Never Allow Relation String Fields To Set Relations To Categories
Relation string fields should never permit to define relations with
categories objects (portal type Category).
The notion of category is not easy to understand for an end user
and having to select a correct category value among a huge list is bad interface.
Category should not be present in the Portal Type field of a relation string field
(example : my_source_title, my_destination_title, my_resource_title field).