`Qt`-in-guest-mode process entry point

[goodboy]

In order to support daemon-rooted-tree spawns of UIs in piker we need to be able to spawn child-actors much like we do with the new infect_asyncio mode but with Qt.

I'm not entirely sure if we should make this a "built-in" thing but at the least we should have a more formal formula for how to build guest-mode entrypoint for such purposes - since there are other UI frameworks and event loops where this same approach is going to be useful. As an example running trio on some kind of rust implemented event-loop runtime in the future I'm sure will be pretty hip.