Previously we used a positional argument for widgets that had multiple
children. Now we use a named argument that defaults to an empty list.

Fixes #241

This package contains WidgetTester, which is very useful when writing tests for
widgets.

- force the time dilation to 1.0 for the Widget tests, so that a local
  change doesn't break all the tests during development.
- add missing license block to all the files.
- set ui.window.onBeginFrame to null when you use WidgetTester, so that
  the engine doesn't trigger any confusing frames after our fake frames.

When we reactivate a subtree that had a global key, we weren't updating the
parent data because:

1) The child wasn't in the tree when we updated the parent data element.
2) The activated child didn't go through mount (just through update).

This patch moves the parent data update work to when we attach the render
object, which we do both during mount and when reactivating a child.

Fixes #345

Fixes #1084

Now that we've ported all the widget tests to fn3, we don't need a separate fn3
directory.