Add rudimentary leak detection to all platforms

[JakeWharton]

We know when things should go away. We know when a widget is removed from the hierarchy. We know when a treehouse app is replaced. In those cases, we can do the world's most basic leak detector of holding those references weakly after removal, and periodically check + force GC to see if they get collected.

Refs #2029. Refs #2033.

[JakeWharton]

This is tracked by the expanded scope of #575