Implement 'Cadence' control blocks for `EntityThread`s

[ImmutableOctet]

These would be control blocks in EntityScript that would change how often the thread is stepped.

I'm currently looking at the following options:

• fixed -- The thread is stepped at a fixed rate per-second. (e.g. 60hz)
• realtime -- The thread is stepped on each realtime update (i.e. each frame)

These would function similarly to multi.

[ImmutableOctet]

This is now implemented in the scene-editor branch.