Rename "Docbase" primitive field type in Document Type Editor

The Document Type Editor provides a primitive field type "Docbase" that can be used in document types. It's basically a String field that lets you pick a content item from the repository, then stores that item's UUID in a String property.

Currently such a reference is not checked when deleting the referenced item (as is done in case of virtual node links).

The name "Docbase" seems very undescriptive to me. Most users won't know what it means until they add a Docbase field and try it out.

New name should cover the following aspects:

• It's a link to another content item.
• It's different from the "Link" compound type.