PrimarySource: Allow cancelling track delete job (bgo#691971)

In the same way the job to add tracks can be cancelled, the
delete one should be as well (this might especially be useful
for device management/sync; although deletion of tracks is
much faster than addition so the user will still have a very
small window to react...).

Signed-off-by: Andres G. Aragoneses <knocte@gmail.com>