Bring Bindings.hpp back in.
Added a few methods to the WindowManipulator interface.
Added a few new event types.
Added Un/registerBinding methods to the WindowManager interface.
Made Default_WindowManager use a more authentic pimpl.
Removed X11XCB_DisplayServer.* for now -- will bring back later.  Added a dummy implementation of the DisplayServer classes.
Use the dummy implementation in the main program.