The Ctrl-Shift-C operator to add constraints between a pair of selected items,
for example, between two objects, or between two bones (in the same armature).
This commit makes it possible to use this operator to add a constraint where the
target is a bone from another object - e.g. to make a deform bone follow the control
bone in another armature, or to make an object use a bone as a tracking target.
Usage:
1) Ensure you are in Pose Mode, then select the bone to use as the target
2) Shift-Select the other object and/or the bone that's going to get the constraint
3) Ctrl-Shift-C
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