rectangle inheritance

Since the current class hierarchy would need to be modified for it to
make sense for drawable to inherit from rectangle, I have moved the
derivation down the tree.  Widget and list_item now derive from
rectangle, not drawable.  I still don't particularly like this solution
either.  Widget inherits from 3 base classes now, which just rubs me the
wrong way for some reason.  If someone has a better solution to the
rectangle issue, please post it.

Darren