UI button: snap joint location to parent/child/center

[htp2]

Hi Adnan - we discussed this in person today, it would be great if we could add 3 buttons (probably at/near the arrow in this picture)

These buttons would set the joint location to be either at the "parent" "child" or "center". Center is just the cartesian center between the two origins.

This is useful because it is easy to accidentally have the origin sit somewhere else (e.g. at the world origin) which then causes joint errors to be much larger than needed due to the relative size of the error terms in those "far-away" joint coordinates