A&O George Giolfan 2008-08-11: Windows.Forms Theming

[George Giolfan]

Achievements
* investigate whether these features can be implemented with GTK+ and
implement them when possible:
* menu rendering; [DONE]
* open file dialog; [NOT DONE]
* save file dialog; [NOT DONE]
* font dialog; [NOT DONE]
* print dialog; [NOT DONE]
* color dialog; [NOT DONE]
* message box; [NOT DONE]
* system colors; [NOT DONE]
* system icons; [DONE]
* window gripper; [DONE]
* separator. [DONE]
Objectives
* clean-up the code from the last two weeks and post it for review;
* discuss the remaining patches and apply those that are accepted;
* test the code on a typical GTK+ theme and perform last-minute fixes
if needed.
Notes
* the native GTK+ dialogs cannot be used because they expect to be
used from a GTK+ application;
* MWF already obtains system colors from GTK+, but only on X11.