Field definitions of document supertypes are not available in subtypes

Since CMS7-5868 it is possible to select a supertype when creating a new document type. However, the fields defined in the supertype are not available at all in the new type. The new document type is always empty. Instead, it should be possible to inherit all (or only some?) of the fields defined in the supertype, e.g. by selecting them from a list.

Note that the supertype and subtype can have a completely different layout, so inheriting fields is not trivial. You may also want to (optionally?) inherit the layout of the supertype. All this probably requires more thought on the desired functionality and UI.