Discard the selection and drag source when not active anymore

If another client sets a selection or starts a drag the active
wl_data_source, if any, gets a 'cancelled' event. We must listen to
this and discard the wl_data_source, else e.g. checking the clipboard
content will still return the data of the source instead of the current
wl_data_offer.

Change-Id: I80b202baf95fbc8abe19496a991cb30bc0b78bb0
Reviewed-by: Gunnar Sletta <gunnar.sletta@jollamobile.com>
Reviewed-by: Robin Burchell <robin+qt@viroteck.net>