[compiz] gnome-window-decorator -> gtk-window-decorator and some restructuring

[David Reveman]

The current name of gnome-window-decorator is wrong. There's nothing
gnome specific in that code. I'll go a head and change the name to
gtk-window-decorator later today.

I'm also going to split the gnome configure option into a gtk and a
gnome option as the only gnome specific code that exist in the tree is
the compiz-window-manager module which is used to integrate with the
gnome control-center. I'll rename the 'gnome' directory in the tree to
'gtk' and move control-panel module into a 'gnome' directory.

Comments, suggestions?

-David