Add Constraint operators are now based on the old add_constraint() function (to be removed when the new code works well).
- Fixed a few bugs with the code, including depsgraph missing rebuild call, and unique-naming for constraints failing due to wrong order for adding constraint to list
- Added capabilities for setting the target of the constraint to the first selected object/bone for the operator when called from menus + hotkeys (but not for buttons window)
This commit is still buggy. I'll fix the remaining issues tomorrow, as well as adding some more operators for IK add/clear.