xwl: Handle X11 clipboard owners closing more thoroughly
Selection acts as an abstraction around multiple X11Sources, when we get a new source we asyncronously emit that x11OffersChanged when it completes.
If the selection is lost, we were just deleting the source, without notifying other parts.
XwlDataSource is the datasource that SeatInterface knows about. We need to delete this when the X11 connection is no longer valid. SeatInterface will update the selection when the XwmlDataSource is deleted if it's the active selection.
The hook is introduced as updating the selection in Clipboard will cause Selection to delete m_xSource which gets messy.