Basically, only meshes, empties and pointclouds support direct
instancing atm., no need to have the panel for other types.
note: prior to rB2eca054e14b1, collection instancing was possible on all
types (but that was removed in said commit)
note2: for empties, rna_Object_instance_type_itemf should also be
tweaked so we dont get "Vertices" and "Faces" options, but that can be
done in a separate commit
