Introduce info/warning/danger to notification area

Error when (auto)saving and things along should stand out in the
UI for the user to investigate the reason as soon as possible not to
loose work later.

Add corresponding styles.

![capture decran 2014-06-29 a 15 39 17](https://cloud.githubusercontent.com/assets/335567/3423364/fa0f6fe6-ff92-11e3-828a-03028e15ea04.png)

![capture decran 2014-06-29 a 15 39 44](https://cloud.githubusercontent.com/assets/335567/3423363/fa0b2120-ff92-11e3-8e89-9b956b44ac4d.png)

![capture decran 2014-06-29 a 15 40 17](https://cloud.githubusercontent.com/assets/335567/3423365/fa134418-ff92-11e3-944d-6f991e1716fe.png)

Should fix #5984

Also fix bugs when clicking on a sticky notification does not dissmiss it.