Update navigation example

Now this example uses the App widget to drive the adventure game.

Also:

- Make Dismissable report when it starts squashing, since otherwise we
  don't invalidate the list early enough and it gets mad that it wasn't
  told one of its children had changed size.

- Have Dismissable check that it gets removed after it's dismissed, to
  avoid having lots of redundant widgets around.

- Start tracking the height of each child of a MixedViewport, so that we
  don't accumulate floating point errors when a child jiggles up and down.

- Have _childOffsets reuse its storage space rather than newing up a new
  copy each time we reset the cache.

- Avoid double-updating child sizes when handling mixed viewport invalidations.

Add a currentWidget and currentState accessor to GlobalKeys

Add ensureWidgetIsVisible to fn3

Renames pointQuickDist in GameMath

Bumped Newton version to 0.1.4

Port some more tests to fn3

Add fling to Scrollable.

Port some more tests to fn3

Port Input and EditableText to fn3

Port even more examples to fn3