Add support for fixed time brightness fading

Do other parts of mce (or its plugins) also set the scheduling parameter? If so, could it be that some other part updates the scheduling parameters, but we don't realize that? E.g.:

1. Scheduling parameters are saved
2. FIFO scheduling is enabled
3. Some other part changes scheduling parameters
4. FIFO scheduling is disabled
5. Saved parameters from (1) are restored (instead of updated parameters from (3))

Also, step (3) should probably only update the saved parameters and not change the priority. In that case, it might make sense to move the whole scheduling parameter code into its own module, so that it can do proper scheduling (sic) of scheduling parameter updates. But maybe that's overkill for now, you decide.