Can not start new fade after all fades have finished

[hynekcer]

This is because the value __fade_running of a universe is cached and never evaluated again if all fades terminated.

A poor fix is possible in dmx_channel.py:

class DmxChannel:
    ...
    def add_fade(self, ...):
        ...
        self.__fade_running = True
        self.__universe._DmxUniverse__fade_running = True  # type: ignore[attr-defined]
        return None

but it is not nice to modify protected attributes. Maybe you add a method to restart processing in a universe.