A maximized window can't be resized from the screen edges (preserves
Fitts law goodness for the application), but it's still possible
to start a resize drag with alt-middle-button. Currently we just
don't let the user resize the window, while showing drag feedback;
it's more useful to let the user "break" out from the resize.

This provides a fast way to get a window partially aligned with
the screen edges - maximize, then alt-drag it out from one edge.

Behavior choices in this patch:

 - You can drag out a window out of maximization in both directions -
   smaller and larger. This can be potentilaly useful in multihead.

 - Dragging a window in only one direction unmaximizes the window
   fully, rather than leaving it in a horizontally/vertically
   maximized state. This is done because the horizontally/vertically
   maximzed states don't have clear visual representation and can
   be confusing to the user.

 - If you drag back to the maximized state after breaking out,
   maximization is restored, but you can't maximize a window by
   dragging to the full size if it didn't start out that way.

A new internal function meta_window_unmaximize_with_gravity() is
added for implementing this; it's a hybrid of
meta_window_unmaximize() and meta_window_resize_with_gravity().