Implement more accurate clock waiting on Windows
Windows may have a possible implementation using Waitable timers but that is not implemented here and instead falls back to the GCond implementation. GCond waits on Windows is still as accurate as the previous GstPoll-based implementation.
Should indeed be possible around
WaitForMultipleObjects (we can't wait just for the timer as that does not allow cancellation: cancelling a waitable timer causes all threads to continue waiting until timeout according to the documentation).