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.

Fixes #1084

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

This argument isn't needed anymore now that State has a getter for context.

This makes the sync code stop if it would have to rearrange the
RenderObjects. I'll make it handle the cross-RenderObject case, as well
as the insertion-sync case, in subsequent patches.

This feature is causing a bug because the widget tree isn't correctly
synchronized with the render tree.

This should handle a case like a stateful component inside a Container
inside another Container having one of those Containers removed while
still keeping that stateful component around with its state.

The problem of how to handle the Container then being reinserted is a
separate issue not handled by this patch.