Add optional automatic WifiLock handling

[ojw28]

This is a feature request for an option to automatically handle WifiLocks (similar to the option for WakeLock handling added in 2.11).

It probably only makes sense to handle (1) neither type of lock, (2) WakeLock only (for local background playbacks) or (3) both WakeLock and WifiLock (for remote background playbacks).