#2 Sound Manager refactoring

[Fabrizio Nunnari]

The sound manager must be refactored according to these new requirements:
- sound sources have to be grouped for scenes
- Decouple interface from implementation: communication with remote SuperCollider engine is needed in future.
- A 3d object must be able to play more sounds at the same time. This is neede to leave echoes and reverberations finish while another sample is staretd.
- There is not really need of a add_audio_source control score command. This can be accomplished transparently. Instead we might want to pre-load the files with a "prepare_audiofile <filename>" command.

[Fabrizio Nunnari]

Partially implemented.
Solved point 3 and 4 of original message.