[WIP] Menu applet/Shell Toolkit:  Add a polygon widget class and implement it in the menu applet

This is to prevent inadvertant category changes when moving the mouse
over to the applications box.

See issue #1775

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.

Not quite ready for primetime - here's a screen cap I made of it in debug mode, to give an idea how it works:

https://dl.dropbox.com/u/8201170/cinnamon-20130311-1.webm

I've still got some tweaking to do, but it's a good start I think.