Limit globally addable types

I think we should limit the addable types on the Plone Site root to the important ones:
![bildschirmfoto 2013-08-05 um 17 05 12](https://f.cloud.github.com/assets/7469/910788/7607e534-fde0-11e2-94b0-9e250c1da881.png)

Which ones should be addable?