Automatically detect mapping type based on the nature of the field

In RDF UI we left it to the site administrator to decide whether the relation is of type property or rel, but now is the time to make that decision automatic. For fields of type text, long text, lists and numbers it should be 'property' (the default), and for file, image, term reference, node reference and user reference, it should be 'rel' (to put it simply, anything that is more than just a string, anything that generates a link to another piece of content).

Entity might help us to be generic here, but that would introduce a dependency. Let's think if there is some ways to avoid entity and is better than a white list of field types.