Hides unrelated tabs from the metadata view

The content cover, references, and history tabs are now hidden.
It is also possible to hide the Categories and Attachment tabs using
InterceptionPoints (SiteNodeVersionEditor.[Categories, Attachments]).

This commit also introduces a setting for defining the content type
to use for Meta infos.
Merge pull request #73 from stenbacka/hide-tabs-in-metadata-view

Hides unrelated tabs from the metadata view