Split up "Play while screensaver is active" into "Play on screensaver activation" and "Pause on screensaver deactivation".

This should ensure that users will get exactly what they want out of these features, sans a little bug with "Pause on screensaver deactivation" caused by upstream (ha ha) http://stackoverflow.com/questions/11106199/get-notified-when-macos-goes-to-sleep