window: Atomically unmaximize both directions from a _NET_WM_STATE ClientMessage

When GDK sends an unmaximize _NET_WM_STATE ClientMessage, it tells us to remove
the _NET_WM_STATE_MAXIMIZED_HORZ and _NET_WM_STATE_MAXIMIZED_VERT states. Before
this time, it would independently call:

  meta_window_unmaximize (window, META_MAXIMIZE_HORIZONTAL);
  meta_window_unmaximize (window, META_MAXIMIZE_VERTICAL);

Which, besides being foolishly inefficient, would also mess up our saved_rect
tracking, causing the window to only look like it was unmaximized vertically.

Make this code more intelligent, so it causes us to unmaximize in one call.

https://bugzilla.gnome.org/show_bug.cgi?id=722108