Some RandomPlay cleanup

- remove unused code and variables
- use $client->pluginData or cache instead of global variables
- simplify variable initialization
- silence a DB warnings
- streamline genre handling: don't re-read from the DB over and over again
- remove references to defunct lastplayed check
- register RandomPlay alarms the standard way
- make year mixes behave the same way as tracks - don't queue up all
tracks at once, respect genre filtering
- don't keep the potentially large idList in memory, but in the cache.
Only read when needed.

Fix bug 7823
Add import from mysb.com feature.
Merge branch 'public/feature/RandomPlay-cleanup' into public/7.8