Display document-icons in ContentTreeWidget

To allow displaying the document mimetype icons in the ContentTreeWidget, like the following picture shows, some rework in the OpengeverNavtreeStrategy was needed: 
 - Drop the `item_icon` attribute for all documents nodes, so it doesn't show a `<img>` tag for documents
 - Reset `normalized_portal_type` to mimetypes css class, leading with a space to avoid the `contenttype` prefixing (see https://github.com/plone/plone.formwidget.contenttree/blob/1.0.5/plone/formwidget/contenttree/input_recurse.pt#L14).  This hack is save because, the `normalized_portal_type` attribute is only used by the contenttree widgets.

![bildschirmfoto 2014-07-09 um 10 22 39](https://cloud.githubusercontent.com/assets/485755/3521624/36e01946-0742-11e4-85e6-0442b5386289.png)

An final solution for this problem should be find, when solve the complete [mimetypes-sprites-icon problem](https://github.com/4teamwork/gever/issues/7).