viewer: make it possible to split recordings back into dedicated windows

[teh-cmc]

(Following a standup discussion)

The current application model (and refinements proposed in #919) make it possible to inspect any number of recordings concurrently in a single Rerun window.
In a way, recordings are akin to tabs in web browsers.

Unfortunately, as of today it is impossible to move any of these recordings back into their own dedicated window (e.g. because you'd like to inspect them side-by-side), similar to e.g. how you'd turn a browser tab into a whole new browser instance.

Implementing such a thing raises even more questions regarding the application and client-server models.

[emilk]

I think just being able to put two recording side-by-side inside the same Rerun window would be a great first step, and quite doable. It would likely force a nice refactor of App too.

[emilk]

This is coming soon:

• Multiple viewports/windows emilk/egui#3172