GL contexts and GC blits don't work, dialog placement doesn't work,
and checkbox animations seem to interefere with updating --- but at
least things mostly work.
Relevant to racket/racket#1547
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.
