The `fullscreen` method implied `show` on Mac OS X and Windows, but
not on Gtk, and a frae shown that was was not properly registered
as shown (e.g., the application could exit). For consistency, adjust
`fullscreen` to not imply `show`. Also, shift to a remaining frame
on Mac OS X when a fullscreen frame is closed.
The main advantage of GTK+ 3 is better support for HiDPI
displays. If GTK+ 3 libraries are not available or if the
`PLT_GTK2` environment variable is defined, GTK+ 2 is used
as before.
