Store the last Weld target attachment

After performing a weld, there are cases where users may want to adjust weights and then run weld again afterward. For example, a user may weld the `eyes` mesh to the `head` mesh, later adjust the weights of the `head` mesh, and then want to weld the `eyes` mesh to the head mesh again so their weights match once more.

To make repeated weld operations faster, it would be useful if each mesh attachment could store the last attachment it was welded to. Then, a feature such as a “Reweld” button could allow users to quickly reapply the previous weld without manually selecting the target attachment again each time.

In addition, if this could also be executed via a hotkey while multiple mesh attachments are selected, it would make it much easier to quickly propagate weight changes from a head mesh to multiple attachments such as `eyes`, `nose`, and `mouth`.

**Related forum thread:**
https://esotericsoftware.com/forum/d/30117-%E6%B7%BB%E5%8A%A0%E5%8A%A8%E6%80%81%E7%84%8A%E6%8E%A5%E5%8A%9F%E8%83%BD/5