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/ui/background.js

@@ -94,7 +94,7 @@
 //     MetaBackgroundImage         MetaBackgroundImage
 //     MetaBackgroundImage         MetaBackgroundImage
 
-const { Clutter, GDesktopEnums, Gio, GLib, GObject, GnomeDesktop, Meta } = imports.gi;
+const {Clutter, GDesktopEnums, Gio, GLib, GObject, GnomeBG, GnomeDesktop, Meta} = imports.gi;
 const Signals = imports.misc.signals;
 
 const LoginManager = imports.misc.loginManager;
@@ -642,7 +642,7 @@ var BackgroundSource = class BackgroundSource {
 };
 
 var Animation = GObject.registerClass(
-class Animation extends GnomeDesktop.BGSlideShow {
+class Animation extends GnomeBG.BGSlideShow {
     _init(params) {
         super._init(params);