(cherry picked from commit afa84db7)

(cherry picked from commit 8e27765c)

Following glade's port to meson the build system in the flatpak
manifest has been also been moved to meson.

The CI-built has also been changed to use it.

meson is a build system focused on speed an ease of use, which
helps speeding up the software development.

meson build system has been added along with autotools.

Both `gtk+` and `webkit2gtk` plugins use input files that are not
actually translated. These input files has been renamed to their
final names.

Also remove an obsolete entry that gettext can’t deal with.

Having data directory paths, like catalogdir, being relative to datadir
allows to resolve them with an arbitrary prefix. Packages installing
Glade catalogs may use this to resolve catalogdir using their prefix.

#353

- GNOME 3.32 redesign of app icons.
  See Initiatives#2
  for more info.

/label ~"9. Initiative: AppIconRedesign"

If you try to load a non-XML file, such as Glade's own README, the
parser error contains something like this:

    Start tag expected, '<' not found

glade_util_ui_message() treats the supplied message as markup; so the
literal < in the error message trips it up:

    Gtk-WARNING **: Failed to set text 'Error parsing file '/home/wjt/src/gnome/glade/README' on line 1
    Start tag expected, '<' not found
    ' from markup due to error parsing markup: Error on line 2 char 30: Odd character 'f', expected a '=' after attribute name 'not' of element '''

To fix this, escape the message before passing it to
glade_util_ui_message().

See https://wiki.gnome.org/Projects/GnomeCommon/Migration

GTK_STOCK_MISSING_IMAGE/"gtk-missing-image" has been deprecated for a
while now and Adwaita does not ship it anymore under that name. Better
prefer the new "image-missing" replacement.

Recent gettext version can extract and merge back strings from and to
various file formats, no need for  intltool anymore.

https://wiki.gnome.org/Initiatives/GnomeGoals/GettextMigration

#279

(cherry picked from commit 31bce15d)

specify dbus slot for gtkapplication registration