post plugin infrastructure for audio:
- when creating a new stream use the null output and rewire it to a real output when it is available
- every AudioPath keeps it's AudioEffects in a AudioPostList object which defines the processing path between xine_stream and xine_audio_port
- started to make multiple AudioPaths per MediaProducer possible (needs another custom post plugin)
- implement AudioEffect::parameterList()
- add virtual function newInstance to AudioEffect that is used by AudioPostList to get a new xine_post_t object of the AudioEffect
- XineStream doesn't take AudioPort objects anymore but AudioPostList objects
- AudioPostList has a function to tell XineStream to start rewiring the post plugins in the xine thread
- implemented VolumeFaderEffect using the KVolumeFader post plugin
- AudioOutput does not create a AudioPort per XineStream anymore - one AudioPort per AudioOutput (output from one source to multiple sinks needs another post plugin)

also:
- no code in Q_ASSERT, this only works for debug builds