Added note about how constraints are represented + create constraints "stack eval" node
Attempted to port motion-tracker constraints to new system

* Most of this seems to port over ok
* Still not sure what exactly the "scene relation" at the end is for
* Because we've got a more descriptive system now, there's probably a lot
  more that we can describe here now that wasn't possible before. However,
  since I'm not familiar with this, I'll leave it as-is for now.
Temporary hack for name used when adding constraint stack op

This is because, this name currently serves double-duty when we're faced
with subdata nodes... This needs urgent fixing!