compositor: Undo MetaWindowDrag on MetaWindow::unmanaged signal
The current usage of MetaWindow::unmanaging may result in confused focus window lookups while undoing the MetaWindowDrag grab (i.e. still pointing to the window that is now being unmanaged).
The meta_window_unmanage() function itself takes care of changing focus outside of the window being unmanaged, so postpone the MetaWindowDrag undoing to a point after that is done.
Fixes: #3073 (closed)