reconsider naming of session::Clip and asset::Clip

[ichthyo]

Generally speaking, within Lumiera Proc-Layer the Assets represent the bookkeeping aspect of things, while the corresponding dynamic aspect is modeled as MObject. But especially with Clips, the similarity of the names turned out to be even confusing for Ichthyo, thus we should think about a more clear naming scheme here.

A possibility would be to rename session::Clip into ClipMO

Explanation: the asset::Clip servers the special purpose to clamp together the multiple media channels forming the channels of a clip, which might be linked together from multiple source media.

[ichthyo]

looks more like we'll drop asset::clip entirely

[ichthyo]

Now decided to purge asset::clip. Thus session::Clip becomes "the" clip