Polybuilder Needs Undo

This tool is too complicated to expect perfection from users. Undo is a sorely needed feature.
This task will take time. Figure out a workflow that can be paused and returned to.

- [ ] Might as well use this as an excuse to insert an observer pattern. Might clarify the entire workflow.
- [ ] Use the observer pattern to cause an Undo_Mngr to react to events by adding them to an ordered list
- [ ] Add an Undo button
- [ ] manual testing needed: (each context) x (each event type) : verify sequence & verify updated position availability