On Unix, expose a way to register file descriptors into the polling loop

Under the hood both Unix backends use calloop, which supports registering arbitrary file descriptors through the polling crate. However, winit usually isn't the only thing running in the GUI system; there's also zbus, accesskit and other things. It would be nice if you could register these systems into winit and poll them without needing to spawn another thread.

Two potential options here:

• Expose a way to register file descriptors in calloop, then receive events as normal through the winit event loop. Then expose a higher-level I/O mechanism in another crate.
• Port from calloop to async-io. This would take a lot more work and be more controversial, but it would allow for seamless integration with zbus.

cc https://github.com/AccessKit/accesskit/pull/337#issuecomment-2002494310, @mwcampbell

Start by reading the two Unix backends and their calloop event-loop integration, then compare calloop's polling support with the proposed async-io approach. The issue requires choosing between these designs before implementation; done would mean a supported way to integrate external file descriptors and receive their events through winit without another thread.