added option in the settings to have the tabs in all windows rotate simultaneously.

Fixed the bug that only the tabs in the currently active window will rotate. Now the tabs will always rotate in the window (and only that one) in which the "rotate tabs" button was selected.

complete port to Web Extension architecture. Additional an hidden option was added to allow for individual times for different tabs. See description for details.
Small bugfix compared to the beta version: invalid error message removed when entering larger rotation times. (thanks to Nick for reporting this)