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>

meson.build

@@ -30,7 +30,7 @@ polkit_req = '>= 0.100'
 schemas_req = '>= 42.beta'
 startup_req = '>= 0.11'
 ibus_req = '>= 1.5.19'
-gnome_desktop_req = '>= 3.35.90'
+gnome_desktop_req = '>= 40'
 
 nm_req = '>= 1.10.4'
 secret_req = '>= 0.18'
@@ -89,7 +89,7 @@ startup_dep = dependency('libstartup-notification-1.0', version: startup_req)
 ibus_dep = dependency('ibus-1.0', version: ibus_req)
 x11_dep = dependency('x11')
 schemas_dep = dependency('gsettings-desktop-schemas', version: schemas_req)
-gnome_desktop_dep = dependency('gnome-desktop-3.0', version: gnome_desktop_req)
+gnome_desktop_dep = dependency('gnome-desktop-4', version: gnome_desktop_req)
 
 nm_deps = []
 if get_option('networkmanager')