Search type filter: limit types to main types.

The type filter is limited to the following types:
* Dossier
* Document/Mail
* Task/Forwarding

Before:
![bildschirmfoto 2014-07-24 um 18 19 52](https://cloud.githubusercontent.com/assets/485755/3691045/62d7bf64-134e-11e4-9029-b7652489e22a.png)

After:
![bildschirmfoto 2014-07-24 um 18 20 28](https://cloud.githubusercontent.com/assets/485755/3691066/7a8fd2b8-134e-11e4-8ea0-87a27e7770e1.png)