Fix compilation / regenerate defs in master

This is probably OK, but since it's the first time I've ever gotten regeneration to work without trashing loads of stuff it obviously shouldn't... I wanted a quick 2nd opinion before merging.

I originally started trying to wrap StyleContext.get_property(), then I found I couldn't compile, then as a follow to making popover.* compile I tried to wrap the moved-from-there gtk_popover_menu_new_from_model()... but I got stuck because it calls the private gtk_menu_section_box_new_toplevel() and I don't know how to proceed, if it's even possible to get around that.

Anyway, hopefully this works and saves anyone else doing this work for a change!