wl_data_offer selection is not destroyed when a window is destroyed

According to the protocol, we should destroy it when losing keyboard focus.

Currently, if a wl_surface is destroyed (which happens when a QWindow is deleted, hidden, or changes type etc.), we do not destroy the offer until we get a new selection.

This means that we might think we still have a valid selection the next time we get keyboard focus, and we're preventing compositor resources from being freed in the meantime.