x11: Cancel _NET_WM_MOVERESIZE if we get a matching ButtonRelease

x11: Cancel _NET_WM_MOVERESIZE if we get a matching ButtonRelease

This implements the following part of the EWMH spec:

"The special value _NET_WM_MOVERESIZE_CANCEL also allows clients to cancel the
operation by sending such message if they detect the release themselves
(clients should send it if they get the button release after sending the move
resize message, indicating that the WM did not get a grab in time to get the
release)."

In particular, it fixes the case of clicking widgets that use
gdk_window_begin_[resize|move]_drag*() and the click "sticking", i.e. the
mouse button getting released but the resize or move operation remaining in
effect.

https://bugzilla.gnome.org/show_bug.cgi?id=669208