docs: Migrate from gtk-doc to gi-docgen

gtk-doc has been slowly dying for the past few years; with gi-docgen we
have a nice successor.

This also makes sure the C documentation also uses the GIR file, which
in turn means faster build times (since all the C code doesn't have to
be parsed and recompiled again), and has a clear dependency graph.

See the [gi-docgen tutorial] for more info on how the system works.

[gi-docgen tutorial]: https://gnome.pages.gitlab.gnome.org/gi-docgen/tutorial.html

devel-docs/reference/gimp-ui/Makefile.am

@@ -0,0 +1,26 @@
+## Process this file with automake to produce Makefile.in
+
+GIMP_DOC_LOGO = $(abs_top_srcdir)/data/images/gimp-logo.ong
+
+gimp-ui-3.0.toml: gimp-ui-3.0.toml.in
+	$(AM_V_GEN) sed \
+		-e 's/@GIMP_VERSION[@]/$(GIMP_VERSION)/' \
+		-e 's/@GIMP_LOGO[@]/$(GIMP_DOC_LOGO)/' \
+		$< $@
+
+# Markdown content files
+content_files = \
+	widget-gallery.md \
+	$(NULL)
+
+GimpUi-reference: $(top_builddir)/libgimp/GimpUi-@GIMP_API_VERSION@.gir $(top_builddir)/libgimp/Gimp-@GIMP_API_VERSION@.gir $(content_files) gimp-ui-3.0.toml
+	$(GI_DOCGEN) \
+	generate \
+	--quiet \
+	--fatal-warnings \
+	--config=gimp-ui-3.0.toml \
+	--output-dir=$(abs_builddir)/Gimp-3.0 \
+	--no-namespace-dir \
+	--content-dir=$(abs_srcdir) \
+    --add-include-path=$(top_abs_builddir)/gimp \
+	$<

devel-docs/reference/gimp-ui/gimp-ui-3.0.toml.in

@@ -0,0 +1,108 @@
+[library]
+namespace = "GimpUi"
+version = "@GIMP_VERSION@"
+browse_url = "https://gitlab.gnome.org/GNOME/gimp/"
+repository_url = "https://gitlab.gnome.org/GNOME/gimp.git"
+website_url = "https://www.gimp.org"
+authors = "GIMP contributors"
+logo_url = "@GIMP_LOGO@"
+license = "GPL-3.0-or-later"
+description = "GIMP UI library"
+dependencies = [
+  'Babl-0.1',
+  'GLib-2.0',
+  'GObject-2.0',
+  'GdkPixbuf-2.0',
+  'Gegl-0.4',
+  'Gio-2.0',
+  'Gtk-3.0',
+  'cairo-1.0',
+]
+devhelp = true
+search_index = true
+
+  [dependencies."Babl-0.1"]
+  name = "Babl"
+  description = "Pixel encoding and color space conversion engine"
+  docs_url = "https://gegl.org/babl"
+
+  [dependencies."GLib-2.0"]
+  name = "GLib"
+  description = "C Utility Library"
+  docs_url = "https://developer.gnome.org/glib/stable"
+
+  [dependencies."GObject-2.0"]
+  name = "GObject"
+  description = "The base type system library"
+  docs_url = "https://developer.gnome.org/gobject/stable"
+
+  [dependencies."GdkPixbuf-2.0"]
+  name = "GdkPixbuf"
+  description = "Image loading and scaling"
+  docs_url = "https://docs.gtk.org/gdk-pixbuf/"
+
+  [dependencies."Gegl-0.4"]
+  name = "Gegl"
+  description = "Generic Graphics Library"
+  docs_url = "https://gegl.org/"
+
+  [dependencies."Gio-2.0"]
+  name = "Gio"
+  description = "GObject interfaces and objects"
+  docs_url = "https://developer.gnome.org/gio/stable"
+
+  [dependencies."Gtk-3.0"]
+  name = "GTK"
+  description = "The GTK toolkit"
+  docs_url = "https://developer.gnome.org/gtk3/stable"
+
+  [dependencies."cairo-1.0"]
+  name = "Cairo"
+  description = "A 2D graphics library with support for multiple output devices"
+  docs_url = "https://www.cairographics.org/manual/"
+
+[theme]
+name = "basic"
+show_index_summary = true
+show_class_hierarchy = true
+
+[source-location]
+base_url = "https://gitlab.gnome.org/GNOME/gimp/-/blob/master/"
+
+[extra]
+content_files = [
+  'widget-gallery.md',
+]
+content_images = [
+  'images/browser.png',
+  'images/busy-box.png',
+  'images/button.png',
+  'images/chain-button.png',
+  'images/color-area.png',
+  'images/color-button.png',
+  'images/color-hex-entry.png',
+  'images/color-notebook.png',
+  'images/color-profile-combo-box.png',
+  'images/color-profile-view.png',
+  'images/color-scale.png',
+  'images/color-scales.png',
+  'images/color-select.png',
+  'images/color-selection.png',
+  'images/dialog.png',
+  'images/enum-combo-box.png',
+  'images/enum-label.png',
+  'images/file-entry.png',
+  'images/frame.png',
+  'images/hint-box.png',
+  'images/int-combo-box.png',
+  'images/memsize-entry.png',
+  'images/number-pair-entry.png',
+  'images/offset-area.png',
+  'images/page-selector.png',
+  'images/path-editor.png',
+  'images/pick-button.png',
+  'images/preview-area.png',
+  'images/ruler.png',
+  'images/string-combo-box.png',
+  'images/unit-combo-box.png',
+]

devel-docs/reference/gimp-ui/meson.build

@@ -0,0 +1,38 @@
+# Extra markdown files
+gimp_ui_doc_content_files = [
+  'widget-gallery.md',
+]
+
+gimp_ui_doc_toml = configure_file(
+  input: 'gimp-ui-3.0.toml.in',
+  output: '@BASENAME@',
+  configuration: {
+    'GIMP_VERSION': gimp_version,
+    'GIMP_LOGO': gimp_logo_dir / 'gimp-logo.png',
+  },
+)
+
+gimp_ui_docs = custom_target('gimp-ui-docs',
+  input: libgimpui_gir[0],
+  output: 'GimpUi-@0@'.format(gimp_api_version),
+  command: [
+    gi_docgen,
+    'generate',
+    '--quiet',
+    '--fatal-warnings',
+    '--config', gimp_ui_doc_toml,
+    '--output-dir=@OUTPUT@',
+    '--no-namespace-dir',
+    '--content-dir=@0@'.format(meson.current_source_dir()),
+    '--add-include-path=@0@'.format(meson.build_root() / 'libgimp'),
+    '@INPUT@',
+  ],
+  depends: libgimp_gir[0],
+  depend_files: [
+    gimp_ui_doc_toml,
+    gimp_ui_doc_content_files,
+  ],
+  build_by_default: true,
+  install: true,
+  install_dir: get_option('datadir') / 'doc',
+)

devel-docs/reference/gimp-ui/widget-gallery.md

@@ -0,0 +1,36 @@
+Title: Widget gallery
+
+Widget gallery
+==============
+
+[![Browser](browser.png)](class.Browser.html)
+[![Button](button.png)](class.Button.html)
+[![BusyBox](busy-box.png)](class.BusyBox.html)
+[![ChainButton](chain-button.png)](class.ChainButton.html)
+[![ColorArea](color-area.png)](class.ColorArea.html)
+[![ColorButton](color-button.png)](class.ColorButton.html)
+[![ColorHexEntry](color-hex-entry.png)](class.ColorHexEntry.html)
+[![ColorNotebook](color-notebook.png)](class.ColorNotebook.html)
+[![ColorScale](color-scale.png)](class.ColorScale.html)
+[![ColorScales](color-scales.png)](class.ColorScales.html)
+[![ColorSelect](color-select.png)](class.ColorSelect.html)
+[![ColorSelection](color-selection.png)](class.ColorSelection.html)
+[![ColorProfileComboBox](color-profile-combo-box.png)](class.ColorProfileComboBox.html)
+[![ColorProfileView](color-profile-view.png)](class.ColorProfileView.html)
+[![Dialog](dialog.png)](class.Dialog.html)
+[![EnumComboBox](enum-combo-box.png)](class.EnumComboBox.html)
+[![EnumLabel](enum-label.png)](class.EnumLabel.html)
+[![FileEntry](file-entry.png)](class.FileEntry.html)
+[![Frame](frame.png)](class.Frame.html)
+[![HintBox](hint-box.png)](class.HintBox.html)
+[![IntComboBox](int-combo-box.png)](class.IntComboBox.html)
+[![MemsizeEntry](memsize-entry.png)](class.MemsizeEntry.html)
+[![NumberPairEntry](number-pair-entry.png)](class.NumberPairEntry.html)
+[![OffsetArea](offset-area.png)](class.OffsetArea.html)
+[![PageSelector](page-selector.png)](class.PageSelector.html)
+[![PathEditor](path-editor.png)](class.PathEditor.html)
+[![PickButton](pick-button.png)](class.PickButton.html)
+[![PreviewArea](preview-area.png)](class.PreviewArea.html)
+[![Ruler](ruler.png)](class.Ruler.html)
+[![StringComboBox](string-combo-box.png)](class.StringComboBox.html)
+[![UnitComboBox](unit-combo-box.png)](class.UnitComboBox.html)