Add experimental egui integration

This commit is authored by @follower, with some minor modifications by
@hannobraun (squashing a lot of commits into one,  running `cargo fmt`,
and auto-applying Clippy suggestions).

This commit re-adds `fj-viewer`'s dependency on winit, indirectly via
`egui`'s winit integration. This is not desirable, but it is acceptable
for now. Getting a working egui integration is worth more.

@hannobraun will open an issue after merging this to track this problem.