feat(ui): add multi-viewport support, convert games window to viewport

[philpax]

Had Crod plug away at this for a bit. It does work, but:

1. Closing the games window does not consistently work. Probably some kind of timing-of-state-manipulation issue.
2. Some of this code seems a little sus.

In any case, it's a good start, I think. Should be easy enough to make this work for the next release.