Build thing more redux

[RolandJansen]

the current API uses objects and methods to create plugins and seqparts. But it could be done with pure redux:

• static action creators for these operations
• reducers that build the state
• registry that describes to the dispatcher and does all the other things.

These root actions could be IOscActions to keep things consistent.

[RolandJansen]

This was done with SeqPart but it needs cleanup and more tests and (more important) more time. Currently I'm busy with getting intermix feature-complete so this thing will be stalled until October/November.