[bp/r47899][FIX][ENH] admin categories: Use the unified search object selector to select objects to add to category as the existing code is deeply flawed. 
[bp/r47958]Add a way of accessing the list of types that can be used as objects (and therefore categorised) - deployment to come next for category admin 
[bp/r47959][FIX][ENH] category admin: Restrict the list of types offered on add to category to those that can be categorised.
This reduces the list so much (no blogs etc) that a new pref seems the best way to do this for now (default=n)