sd::slidesorter::controller::Listener::Notify(): when the ~SdDrawDocument
sends a HINT_MODELCLEARED, unregister since the document doesn't exist
any more. This should hopefully avoids accessing a SfxViewShell
instance that has apparently already been deleted somewhere in one of
the listeners.
Change-Id: I0ba9a58d77741a52b652a6c55c2327db80859011