Update to drop GTK3 dependency

There's a few things going on here, that unfortunately must
happen in lockstep:

- The gnome-desktop-3.0 dependency gets replaced by gnome-desktop-4
  and gnome-bg-4. The code in ui/background.js required minor updates.
- The libnma dependency gets replaced by a libnma4 dependency. The
  code in misc/modemManager.js required minor updates.
- The gtk3 dependency is torn down everywhere but tests. Some
  missing GdkPixbuf dependencies had to be added to compensate for
  its lack.
- gtk_init_check() is no longer called

As a result, we replace a hard gtk3 dependency with a soft gtk4
run-time linking one, only added indirectly through gnome-bg-4
and libnma4.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2655>

js/misc/modemManager.js

@@ -1,7 +1,7 @@
 // -*- mode: js; js-indent-level: 4; indent-tabs-mode: nil -*-
 /* exported ModemBase, ModemGsm, ModemCdma, BroadbandModem  */
 
-const { Gio, GObject, NM, NMA } = imports.gi;
+const {Gio, GObject, NM, NMA4} = imports.gi;
 
 const { loadInterfaceXML } = imports.misc.fileUtils;
 
@@ -14,7 +14,7 @@ let _mpd;
 function _getMobileProvidersDatabase() {
     if (_mpd == null) {
         try {
-            _mpd = new NMA.MobileProvidersDatabase();
+            _mpd = new NMA4.MobileProvidersDatabase();
             _mpd.init(null);
         } catch (e) {
             log(e.message);