implement base Relationship

[tmpvar]

The concept here is that we can bind points/lines/etc to other things by a variety of relationships

api surface

Relationship(owner)

add(thing)

• watch thing.change
• apply the appropriate changes to thing (if possible!)

note: applying change to thing might be blocked by other relationships.

remove()

apply the inverse of add

render()

Provide a way to visualize the relationship

future

These relationship add/removes need to integrate with the undo/redo manager.