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

git-svn-id: file:///var/local/code.tiki.org/tikisvn/trunk@47958 b456876b-0849-0410-b77d-98878d47e9d5
[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)

git-svn-id: file:///var/local/code.tiki.org/tikisvn/trunk@47959 b456876b-0849-0410-b77d-98878d47e9d5