This adds an optional idle shutdown timer which can be enabled
via timers.idle_shutdown.timeout_sec in the jukebox.yaml config.

The system will shut down after the given number of seconds if no
activity has been detected during that time. Activity is defined as:
- music playing
- active SSH sessions
- changes in configs or audio content.

Fixes: MiczFlor#1970

… and GenericEndlessTimerClass timers