Requesting a selection with a NULL data source means "unset the clipboard",
but internally we use an unset clipboard as the indication that the
clipboard manager should take over.
Moreover, this unset request may go unheard if the current owner is someone
else than the MetaWaylandDataDevice.
Instead, set a dummy data source with no mimetypes nor data, this both
prevents the clipboard manager from taking over and ensures the selection
is replaced with it.
The MetaSelectionSourceMemory was also added some checks to allow for this
dummy mode.
Fixes: https://gitlab.gnome.org/GNOME/mutter/issues/793
