libgimp: 'gimp_ui' as priority prefix for GimpUi introspected module.

Since meson 0.43.0 (below our current requirement), 'symbol_prefix'
argument of gnome.generate_gir() allows an ordered list. If I prepend
'gimp_ui', it makes any gimp_ui_*() function to not start with 'ui_'.

In particular, GimpUi.ui_init() becomes GimpUi.init() which is much less
redundant.

plug-ins/python/palette-offset.py

@@ -134,7 +134,7 @@ class PaletteOffset (Gimp.PlugIn):
             gi.require_version('Gtk', '3.0')
             from gi.repository import Gtk
 
-            GimpUi.ui_init ("palette-offset.py")
+            GimpUi.init ("palette-offset.py")
 
             use_header_bar = Gtk.Settings.get_default().get_property("gtk-dialogs-use-header")
             dialog = GimpUi.Dialog(use_header_bar=use_header_bar,