keep track of attached tethers

I'm finding that it's useful to keep track of Tethers that are tethered to a Tether, e.g. a Drop within a Drop.

This PR adds two properties to Tether instances:
* `attachedTethers`: Tethers whose `target`s are contained by this instance's `element`
* `tetherAttachedTo`: Tether whose `element` contains this instance's `target`