code up timeline and sequence fabrication

[ichthyo]

As result of the design work, there is now a skeleton in the factory functions (of StructFactory, that is).
Anyhow, really need to push this ahead, it blocks a lot of further work on the session interface

[ichthyo]

After investigating my own design notes and the started code, the following is clear:

• we can separate into a required-right-now implementation and a full implementation
• the latter contains the rather tricky parts with magic attachments and ScopeTrigger
• while for the integration it is sufficient to be able to create a Sequence explicitly

Thus, basically the implementation for Timelines is already settled, but for Sequences I need to get a track and thus need to care for simplifying track creation, i.e. removing the track-assets, even if there is no need to build the full ID registration right now.

[ichthyo]

work on this is stalled and postponed a bit -- currently focussing on engine and playback