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>

src/main.c

@@ -10,7 +10,6 @@
 
 #include <cogl-pango/cogl-pango.h>
 #include <clutter/clutter.h>
-#include <gtk/gtk.h>
 #include <glib-unix.h>
 #include <glib/gi18n-lib.h>
 #include <girepository.h>
@@ -474,12 +473,8 @@ list_modes (const char  *option_name,
 
   /* Many of our imports require global to be set, so rather than
    * tayloring our imports carefully here to avoid that dependency,
-   * we just set it.
-   * ShellGlobal has some GTK+ dependencies, so initialize GTK+; we
-   * don't really care if it fails though (e.g. when running from a tty),
-   * so we mute all warnings */
+   * we just set it. */
   g_log_set_writer_func (shut_up, NULL, NULL);
-  gtk_init_check (NULL, NULL);
 
   _shell_global_init (NULL);
   global = shell_global_get ();