Allow implementing a GAction interface #4151

Job	Run time
build GTK binaries with gvsbuild	12s
build gtk-rs on Windows (cairo, true, --features png,pdf,svg,ps,use_glib,v1_16,freetype,script,wi...	3m 4s
build gtk-rs on Windows (gdk-pixbuf, true, --features v2_42)	2m 52s
build gtk-rs on Windows (gio, true, --features v2_80)	3m 44s
build gtk-rs on Windows (glib, true, --features v2_80)	4m 12s
build gtk-rs on Windows (glib-build-tools, false)	2m 21s
build gtk-rs on Windows (graphene, false)	2m 40s
build gtk-rs on Windows (pango, true, --features v1_52)	2m 18s
build gtk-rs on Windows (pangocairo, true)	3m 10s
build gtk-rs on Windows (examples, false, --bins --examples --all-features)	2m 42s
	27m 15s

Provide feedback