gtk4x: Replace gtk4 references with gtk4x/gtk5
Replace references to gtk4 in gtk4x-client with gtk4x or gtk5 depending on context; the client is named gtk4x for now, but it lives under gui-gtk-5.0.
Replace references to gtk4 in gtk4x-client with gtk4x or gtk5 depending on context; the client is named gtk4x for now, but it lives under gui-gtk-5.0.