mostly fix streamreader for refactored pipeline

- cleanup & merge stop with unlock
- fix streamer unlocking
- return if the reader got unlocked before the lock was acquired
- always return right away, to reduce waiting time for possibly waiting
functions
- turn on seekability
- make streamreader a pipeline member supposed to be reused after stop
.
gets thrown away on setting a new source because otherwise libphonon
likes to assert out with the present streamer implementation
- stop reader also on stop()