API to know when to call `uiMainStep`

Instead of calling `uiMainStep` on some arbitrary timer, provide a way know when to step next.

I'd suggest a function that returns both:
1. a file descriptor (unix) or `HANDLE` (windows) to wait on
2. a timeout

These would then be passed (by the user) to `poll`/`select`(unix)/`WaitForMultipleObjects`(windows) to multiplex the UI with other (e.g. network) resources.