Blender Git Loki

Add Custom Object Space to Constraints

Add Custom Space to the list of space conversions for constraints.

Constraints can use World Space, Local Space, Pose Space, Local with
Parent, and now also Custom Space with a custom object to define the
evaluation space.

The Custom Space option uses the Local Space of an other
object/bone/vertex group. If selected on owner or target it will show a
box for object selection. If an armature is selected, then it will also
show a box for bone selection. If a mesh object is selected it will show
the option for using the local space of a vertex group.

Reviewed By: #animation_rigging, sybren, Severin, angavrilov

Differential Revision: https://developer.blender.org/D7437