Videoplayer refactoring, channel layout resampling

 - Split video player to separate source files.
 - Remove MWSound dependencies from the video player. 
 - Move video player engine sources to extern/ (repository will be set up on github soon).
 - Audio is handled in a MovieAudioFactory, implemented by the user (here in MWSound subsystem).
 - Handle conversion of unsupported channel layouts via ffmpeg's swresample.