Add support for interactive animations

[NickEntin]

Adding support for interactive animations will greatly increase the variety of use cases where Stagehand can be applied.

The exact requirements for what this feature should support is still an open question. At this point we're collecting data on what use cases we want to support, so we can design the API around that.

[NickEntin]

Since there isn't actually anything breaking about this change (all of the breaking changes were landed as prerequisites to this) and progress has been a bit slow, I'm going to remove it from the 4.0 milestone. We can release interactive animations as a minor release once it's ready.