rework Asset/MObject design

[ichthyo]

Recent work on the session API highlighted some inconsistencies in the initial design of the Asset hierarchy, especially when it comes to the relation to the MObjects. While the current version is good enough to work towards a first integration of the complete render path, these problems should be rectified during Alpha, to prepare for supporting virtual clips, multiple timelines and sequences.

• the duplication on some entities (clip, track) should be removed
• a general registration mechanism should be used instead of ad-hoc relations
• multichannel support should be pushed mostly into asset::Media and subclasses
• the handling of tracks, sequences and asset folders should be integrated

[ichthyo]

obviously my job, thus I take it, pushing it to future for now.