RFC: remove bookmarks in the "add resources" window

[Fania Bremmer]

** Proposed Change **
Remove the category bookmarks in the "add resources" window.

** Reason For Change **
There is no usecase for adding the same bookmark in every activity again 
and again, in my opinion.
Via the Connect from SLC its very easy and comfortable to add a bookmark 
to my current activity. This is by far the quickes way. Bookmarks and 
websites are subject related. So if a user linkes bookmarks about Qt to 
an activity, it might be gathered in a QTprojectActivity anyway. Why 
should he be intersted in adding this bookmark to another activity? Or 
bookmarks about Prag, because I will travel there... not relevant in my 
Tennis activity.

** Implementation Details **
- remove the whole section "bookmarks" in the add resource window