Fix minor deficiencies

I've done my first build with GTK4 today and this is what I came out of it.

This MR fixes two bugs in the module sets:

• The dependency to gtk-4 is incorrectly specified as gtk-4.0 in a few places.
  -> Change these occurrences to gtk-4.
• My first build attempt failed with meson complaining about missing gdk-pixbuf:

  [...]
  Run-time dependency pangoft2 found: YES 1.48.4
  Run-time dependency pangocairo found: YES 1.48.4
  Found CMake: /Users/Shared/work/0.1/bin/cmake (3.20.0)
  Run-time dependency gdk-pixbuf-2.0 found: NO (tried pkgconfig, framework and cmake)
  Not looking for a fallback subproject for the dependency gdk-pixbuf-2.0 because:
  Use of fallback dependencies is disabled.
  
  ../../../usr/src/gtk-4.2.0/meson.build:385:0: ERROR: Dependency 'gdk-pixbuf-2.0' is required but not found.

  -> Add gdk-pixbuf as depencency to gtk-4.