Add a mechanism to walk the Widget tree

[JakeWharton]

We want parents to be able to act on the entire subtree. We have this at the protocol-layer, but we want it at the Widget layer.

Ideally it's something on Widget, like an allChildren: List<Widget.Children<W>> or maybe even allChildren: Sequence<Widget<W>>.

However, because of Objective-C export, we cannot add generated functions to a Widget sub-interface as they appear unimplemented in the header. So we're blocked by #2231.