Nothing about it is specific to A/B Street, and other apps built with
widgetry could organize themselves as a stack of states. This is also a
first step towards sharing more common code between A/B Street and a
future OSM viewer.
Mostly mechanical change. Some more cleanup / documentation coming up
next.
When I added web support in February, it was easier to get started with
stdweb, since it has the nice cargo web tool. However, stdweb is
unmaintained, winit is deprecating support for it, and the next steps
for web (downloading maps dynamically) have better support for web-sys.
With Alvin's guidance, I got
https://github.com/dabreegster/minimal_websys_winit_glow_demo working
first. This PR cuts A/B Street over too.
I tested abst and the widgetry demo in both native and web. The only
major regression from stdweb is the canvas placement and size. I
attempted some fixes, but at this point, I'll leave it as a smaller
followup instead.
