[test] finished desktop notifications feature
 - desktop notifications
  - must be enabled in options
  - sends notification when period change is detected
NOTE: currently only runs on schedule update, so by default won't run while page is in the background
fixed desktop notifications
(was previously checking day change instead of period change)
various fixes for desktop notifications
 - removed notifications for passing periods
 - added notifications for period start (was previously only end)
 - added safety checks for browsers that don't support notifications api
 - removed debug code
added option to auto-close desktop notifications; fixed notifications
 - added option to automatically close desktop notifications after a given amount of time (0 to disable auto-close)
 - fixed notifications not displaying