Add spawn_future_with_priority and spawn_future_local_with_priority #3120

Job	Run time
build GTK binaries with gvsbuild	39s
build gtk-rs on Windows (cairo, true, --features png,pdf,svg,ps,use_glib,v1_16,freetype,script,wi...	3m 17s
build gtk-rs on Windows (gdk-pixbuf, true, --features v2_42)	3m 11s
build gtk-rs on Windows (gio, true, --features v2_74)	3m 46s
build gtk-rs on Windows (glib, true, --features v2_74)	4m 33s
build gtk-rs on Windows (glib-build-tools, false)	1m 29s
build gtk-rs on Windows (graphene, false)	2m 30s
build gtk-rs on Windows (pango, true, --features v1_50)	3m 10s
build gtk-rs on Windows (pangocairo, true)	2m 25s
build gtk-rs on Windows (examples, false, --bins --examples --all-features)	4m 4s
	29m 4s

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