fix(tooltip): leaking watchers on toggling (WIP)

Make sure to use a new child scope every time as watchers leak into
scope. If linked DOM is removed, watchers from directives in that DOM
aren't removed.

Regression from #1455

This seems like a tough one to add a test for besides counting the number of total watchers. Perhaps that's a good strategy. I'll keep that in mind and possibly add that tomorrow. Heading to sleep.