dbus: duplicate Job.Cancel() as CancelJob() and Snapshot.Remove() as RemoveSnapshot...

dbus: duplicate Job.Cancel() as CancelJob() and Snapshot.Remove() as RemoveSnapshot() on the Manager interface

For all other object mehtods there are already counterparts on the
manager object, as they help us reduce round-trips. So let's complete
this, and reduce complexity on the client side a bit.

As a side effect this also makes "systemctl snapshot" without arguments
work again.