[NC] Add constraint 'mirror' and 'look at' toggles (#64)

Adds the features described here (in a different repo).

Adds the ability to mirror the contraint and to make the child look at the position of the parent.
To make updating easier I also moved the actual contraining logic to the Contraint class itself, and just call those methods. Since the contraint gets updatated in two places with the same logic.

Also fixes the deault values of the scale offsets to be 1 (which was already set to 1 when pressing the reset button) to get normal behaviour inverting the scale by default (does not effect the normal scaling constraint)