app, tools: allow creating demo "scenarios" from the AppStream metadata.

The idea is to add some "demo" attribute to a list item inside the
<release> tag, since we already decided that (for now at least) we'd
keep a strict "intro + list" logics, as we did until now.

This demo attribute uses an internal format to specify successive
widgets to blink (like a demo path towards a feature). For now, what it
allows is:

* raise the toolbox, select a tool and blink the tool button.
* raise a dockable, blink any widgets in there.

Now it is still limited and needs to evolve. In particular:

* What happens if the blinked tool button was explicitly removed from
  Preferences? Should we re-add it for the demo? And once done, should
  we remove it again? Then should we select back the tool previously
  selected?
* What happens if the dockable widget is not visible? Should we allow
  changing the settings to be able to demo correctly the new/changed
  settings? Should it be temporary? If temporary, it can be annoying as
  you'd still have to look attentively the demo to find back the path to
  the new settings. If not temporary, some people may dislike we touch
  their settings.
* What if docks are hidden? Should we unhide them, then hide them back
  after demo time?

Also regarding the implementation: originally I wanted to just grab the
demo attribute directly from the AppStream metadata file, but I realized
that appstream-glib cleans out unknown attribute from the XML. I could
then simply parse the file with a generic XML parser, but I found
simpler to pre-parse it into a header built within GIMP. I still use
appstream-glib at runtime as it takes care of localization for us
(though in the same time, we also have the localization in the main po
files, so maybe we could just embed the release note strings as well).

See appstream-glib report: https://github.com/hughsie/appstream-glib/issues/431

tools/generate-welcome-dialog-data.py

@@ -0,0 +1,114 @@
+#!/usr/bin/env python3
+
+"""
+generate-welcome-dialog-data.py -- Generate app/dialogs/welcome-dialog-data.h
+Copyright (C) 2022 Jehan
+
+This program is free software: you can redistribute it and/or modify
+it under the terms of the GNU General Public License as published by
+the Free Software Foundation; either version 3 of the License, or
+(at your option) any later version.
+
+This program is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+GNU General Public License for more details.
+
+You should have received a copy of the GNU General Public License
+along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+
+Usage: generate-welcome-dialog-data.py
+"""
+
+import argparse
+import os.path
+import sys
+import xml.etree.ElementTree as ET
+
+tools_dir   = os.path.dirname(os.path.realpath(__file__))
+desktop_dir = os.path.join(tools_dir, '../desktop')
+outdir      = os.path.join(tools_dir, '../app/dialogs')
+
+infile      = os.path.join(desktop_dir, 'org.gimp.GIMP.appdata.xml.in.in')
+outfile     = os.path.join(outdir, 'welcome-dialog-data.h')
+
+def parse_appdata(infile, version):
+  release_demos = []
+  tree = ET.parse(infile)
+  root = tree.getroot()
+  releases_node = root.find('releases')
+  releases = releases_node.findall('release')
+  for release in releases:
+    if 'version' in release.attrib and release.attrib['version'] == version:
+      items = release.findall('./description/ul/_li')
+      for item in items:
+        demo = None
+        if 'demo' in item.attrib:
+          demo = item.attrib['demo']
+        release_demos += [demo]
+
+  return release_demos
+
+if __name__ == "__main__":
+  parser = argparse.ArgumentParser()
+  parser.add_argument('version')
+  parser.add_argument('--header', action='store_true')
+  args = parser.parse_args(sys.argv[1:])
+
+  top_comment = '''/* GIMP - The GNU Image Manipulation Program
+ * Copyright (C) 1995 Spencer Kimball and Peter Mattis
+ *
+ * welcome-dialog-data.h
+ * Copyright (C) 2022 Jehan
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ *
+ ***********************************************************************
+ * This file is autogenerated by tools/generate-welcome-dialog-data.py *
+ ***********************************************************************
+ *
+ * Modify the python script or desktop/org.gimp.GIMP.appdata.xml.in.in
+ * instead of this one
+ * Then run tools/generate-welcome-dialog-data.py again.
+ */
+
+'''
+  print(top_comment)
+
+  demos = parse_appdata(infile, args.version)
+
+  if args.header:
+    print('#ifndef __WELCOME_DIALOG_DATA_H__')
+    print('#define __WELCOME_DIALOG_DATA_H__\n\n')
+
+    print('extern gint          n_gimp_welcome_dialog_demo;')
+    print('extern const gchar * gimp_welcome_dialog_demo[];')
+
+    print('\n\n#endif /* __WELCOME_DIALOG_DATA_H__ */')
+  else:
+    print('#include "config.h"')
+    print('#include <glib.h>')
+    print()
+
+    print('const gint    n_gimp_welcome_dialog_demo = {};'.format(len(demos)))
+    print()
+    print('const gchar * gimp_welcome_dialog_demo[] =')
+    print('{')
+    for demo in demos:
+      if demo is None:
+        print('  NULL,')
+      else:
+        print('  "{}",'.format(demo))
+    print('  NULL,\n};')