Add windows.onFocusChanged to stop polling windows not in focus.
Fix double polling issue, now ensure only 1 polling process exists.
Add some debugging.
Un-register youtube tabs that are being destroyed.
Update script state variables to start with underscore for ease in reading where they come from.
Updated state change test to include buffering as a play state, and limited sending videoStarted message to when it goes from a non-playing state to a playing state.
Removed tabs permission, as it seems it's not necessary.