docs: `construct` attribute for `glib::Properties` #3451

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 8s
build gtk-rs on Windows (gdk-pixbuf, true, --features v2_42)	1m 55s
build gtk-rs on Windows (gio, true, --features v2_80)	2m 40s
build gtk-rs on Windows (glib, true, --features v2_80)	3m 15s
build gtk-rs on Windows (glib-build-tools, false)	1m 48s
build gtk-rs on Windows (graphene, false)	1m 47s
build gtk-rs on Windows (pango, true, --features v1_50)	2m 9s
build gtk-rs on Windows (pangocairo, true)	1m 45s
build gtk-rs on Windows (examples, false, --bins --examples --all-features)	2m 4s
	19m 47s

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