Add some quality-of-life gdbus helpers #3980

Job	Run time
build GTK binaries with gvsbuild	16s
build gtk-rs on Windows (cairo, true, --features png,pdf,svg,ps,use_glib,v1_16,freetype,script,wi...	2m 41s
build gtk-rs on Windows (gdk-pixbuf, true, --features v2_42)	1m 37s
build gtk-rs on Windows (gio, true, --features v2_80)	2m 51s
build gtk-rs on Windows (glib, true, --features v2_80)	3m 9s
build gtk-rs on Windows (glib-build-tools, false)	1m 19s
build gtk-rs on Windows (graphene, false)	1m 40s
build gtk-rs on Windows (pango, true, --features v1_52)	2m 45s
build gtk-rs on Windows (pangocairo, true)	3m 12s
build gtk-rs on Windows (examples, false, --bins --examples --all-features)	1m 47s
	21m 17s

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