GTK3 event loop broken on Mac OS X

Jeff Sparkes, Aidan: If you're listening, I'd appreciate an ACK even if you have nothing to say. (Otherwise I'll eventually spam you personally ....) All, On Mac OS X "High Sierra" I'm having trouble with the recently committed GTK3 port (thanks Jeff and Aidan!) Lots of things are less than copacetic, but the first problem I ran into is an infloop if I even mouseover the toolbar. Turns out that gdk_events_pending can return non-zero even when gdk_event_peek returns NULL. So I wrote the attached patch to use the latter to test for no more events. This isn't great, but it seems to work. I think I got the GdkEvent cleanup right, but I suppose it's possible this leaks GdkEvents, so be warned. Steve