racket/gui Gtk: `choice%` and `combo%` repair for some Gtk variants

Finishes repair started in 8cbcba8ed so that mouse events can be
intercepted for `combo%` and `choice%` objects. DrRacket's
font preference choice relies on intercepting mouse events
to populate the menu, for example.

Reference counting doesn't seem to work, probably because the
relevant widgets are supposed to be private.