More GNOME Theme Improvements

Bean running with the GNOME native theme

As you can see...  I'm making some progress.   I've corrected a number of problems with GNUstep's in window menu generation.   It's now very close to what most other apps generate in GNOME.

I considered a few approaches, one was to leverage some of the code I wrote for the Windows theme to generate the menu from the menu structure and create a GtkMenu to attach to the Window... the issue with this approach was that it would have required the window to be a GtkWindow, which wasn't a good idea.  The other approach I considered was to reparent the GNUstep created window into a GtkWindow which contains the GtkMenu.  The issue with this was that it would have been impossible to switch back to another theme which doesn't use the GNOME widgets and GtkWindow.

The approach I settled on is a good middle ground.   The changes fix a number of issues which were present in the code previously.   Also, it gives more control to us regarding the look of the in-window menus.

My plans are to work out the remaining bugs with this and release it so that people can start playing with it.   I will have more screenshots in the future as it becomes more mature.  Feel free to comment on anything you think can be improved. :)