Added more informative toolbarbutton tooltip. Fixes #1868

Fixes https://github.com/greasemonkey/greasemonkey/issues/1868

Hover over Greasemonkey icon to get tooltip:
![tooltip active 2](https://f.cloud.github.com/assets/55841/2121425/53235c08-91e5-11e3-8894-300119f84045.jpg)
![tooltip deactive](https://f.cloud.github.com/assets/55841/2121426/563f65f8-91e5-11e3-886d-4592466156be.jpg)