Automatically show tooltip when moving over term

Didn't manage to add this in #138:

> What's missing is the "show tooltip when moving the cursor over a term" feature (right now you still need to click). From what I can tell this requires a bit of refactoring: 1) we should probably add events for entering/exiting a highlightable div. 2) these need to be hooked up so that they can cause a rerender (which I believe is handled with the call_event wrapper but I don't fully understand this part yet).