Menu applet/Shell Toolkit:  Add a polygon widget class and implement it in
the menu applet to prevent inadvertant category changes when moving the mouse
over to the applications box.

The polygon is drawn and acts like a mask, preventing selection changing while
your mouse movement is towards the app box.

It is normally invisible but can be made visible to help debugging
Have box follow mouse, so we don't get trapped.  (Still need work)
A bit more tweaking.  I'm pretty satisfied now.  Actually works better when
debug mode is off - you're not 'trying' to follow the shape.
A little less category overlap
Fix a category selection problem (not strictly related)
Turn off debug, destroy mask box when the menu closes
Some more optimizations
turn off debug
Make mask more responsive, reduce mask area (works better with the delayed
update, and easier to change categories)