Allow ObWindow subclasses to redirect a child window of the top-level window

Allow ObWindow subclasses to redirect a child window of the top-level window

This means ObClient redirects the client window itself, rather than the
 parenting frame window.
Added a "toparea" to the ObWindow that tracks the top-level window's area,
 so the "area" can track the redirected window's area inside the top-level
 window.
Added window_set_top_area() that must be called before window_set_abstract()
 for subclasses of ObWindow.  This sets the initial position/size/border of
 ObWindow's top-most window, that will be given in window_set_abstract().
 After that, it can track any changes made to the window's geometry on its
 own.

Decor is now invisible since it needs to be drawn by the Compositing code
 explicitly.