Support closing a session

[BartoszKlonowski]

Closing a session would be quite useful.
The closing should work as follows:

1. Select a session from the list, or type it's name
2. Click "Close" with the selected session
3. Add-on will go through all open tabs and
   Check for the tab's ID
   Check for the tab's domain or URL (to be yet decided)
   Compare the above with tabs in session's memory
4. Close the tab if match