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
a6c4e39876