* WIP
* Implement alsapcm_polldescriptors_revents and alspcm_avail
* Open/close the playback device when idle.
It takes a long time until it's stopped, though.
* open/close logic of playback device
* Fix opening logic, make period size divisible by 6
* Be less verbose in level info
* Extra argument for output mixer card index
Sometimes, this cannot be deduced from the output device
* Better silence detection
* Run run_after_stop when idle on startup
The Reactor now takes a callable, and the loopback and volume forwarder are now
implemented as callable instances, which seemed the most Pythonic solution.
This needs the patches from (probably) https://lkml.org/lkml/2021/3/1/419. They are already in the raspberry OS kernel sources and the setup works on a RPi 4
