Refactor video player engine to get rid of MWSound dependencies

 - Split video player to separate source files.
 - 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.
Add missing header to CMakeLists, remove stale comment
namespace fix, improve comment
avresample wrapper fix
Merge remote-tracking branch 'scrawl/master'