GtkFontSelection is available in GTK+3
Oh, ok, deprecated in 3.2
IIRC it's still used in some parts of Geany even in GTK+ 3 (prefs dialog font buttons)
Not here, maybe in some plugins I don't use (and therefore don't care).
Well, showing only the sensible fonts in either dialog is the least suckiest way to go, and then as I said the GTK2 one is downgraded to the GTK3 one anyway so it doesn't matter. As always just needs somebody to do it.