This is a squash of a bunch of wip commits:

Starting on multi-window

Most flow is in place, but not wired to actual window creation yet.

Fix lifetime issues

These only show up on Windows (DirectWrite backend).

Wire window size propagation

Events on window size need to be propagated to the WindowPod so they
can be available during layout.

There are other ways to do this. Instead of being in WindowPod, maybe
the size info should be in WindowState (where prev_paint_time is, for
similar reasons), then passed down through LayoutCtxRoot.

But this way, at least the RootWidget gets notifications of window size
changing, which seems potentially useful.

Small progress towards working multi-window

Get new window containers working

Build the hello example using containers suitable for multi-window.

Still WIP, there's no way to create new windows yet.

Starting to plumb creation of multiple windows

This is basically working but has a few rough spots. One missing feature
is an event flow for closing windows, so their state can be cleaned up.

This removes WindowPod and bakes in the simplest case.

This might be leaking memory, but that beats a crash in the near-term.

- Remove PartialEq for Command/Selector
- remove `pub` from Selector.object
- rename send_ to do_ in win_handler
- cleanup some comments & remove some dead code