Make xdg-shell app_id match application id

See https://developer.gnome.org/gtk4/unstable/gtk-migrating-3-to-4.html#id-1.6.4.3.12

gajim/application.py

@@ -177,7 +177,7 @@ def __init__(self):
 
         self.interface = None
 
-        GLib.set_prgname('gajim')
+        GLib.set_prgname('org.gajim.Gajim')
         if GLib.get_application_name() != 'Gajim':
             GLib.set_application_name('Gajim')