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
It seems that the right answer was produced in GTK+ 2 due to a
client-location callback, but change the code to use the intended
approach of tracking the menu height.
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.
Support GUI scaling in much the same way as on Windows, where
the OS setting ("org.gnome.desktop.interface.scaling-factor"
times "...text-scaling-factor") determines the scale that is
used for both graphics and GUI sizing.
As I understand it, a complete solution requires porting to
Gtk3. With Gtk2, the graphical part of a widget doesn't scale.
Text and image labels should scale correctly, though.
