[transfer-engine] Prevent premature exits

If TransferEnginePrivate::exitSafely() finds out that activity is still
ongoing it should cancel previously sheduled delayedExitSafely() call.
Otherwise delayedExitSafely() may be invoked between two syncs causing
nemo-transfer-engine to exit and get immediately restarted by D-Bus which
isn't something we really want. We want to exit when all the activity has
really calmed down.