addon status indicator (on / off) is changed from being part of the addon icon to being rendered separately via addon badge in order to improve visibility, since Firefox renders icons poorly due to their very small size

Changed how the addon handles URL checks, tab closing and displaying notifications, by adding a queue system.

• Added a queue for URL checks. Previously the addon would perform a check on each URL change and attempt to close duplicates immediately. This could slow down the browser, if tabs are being closed at the same time as you are opening new ones. Now the URL checks are queued, and performed 3 seconds after the last URL change.
• Added a queue for notifications to display them in a staggered manner, this avoids situations where several notifications are fired at the same time and expire at the same time, with the user being able to only see the first one before all of them expire.