[racket-dev] Gtk menu-bar height

I've pushed a new repair to the old menu-bar problem in Gtk, where the
menu bar could be too short to properly display its content.

Robby previously pushed a repair that made the menu bar the right
height when it's supposed to be in a frame. For Unity-like modes where
the menu bar is not included in a frame, however, Robby's patch left a
blank space at the top of the frame. The new patch is meant to fix that
problem.

Let me know if you see any further problems.