Fix puzzle window resize behaviour on Unity.

Fix puzzle window resize behaviour on Unity.

Unity hijacks the menu bar and prevents it from appearing in the main
puzzle window. And we wait for the menu bar to appear before reducing
the puzzle drawing area's size request. These two behaviours go
together badly.

Fixed by detecting the extra GTK property that the Unity patches
invented, and using that to know whether to expect the menu bar to
show up at all.