Child types of custom document types cannot be instantiated

When you create a new document type in the configuration editor that has a supertype that is one of your document types, and not the basedocument, the resulting document cannot be instantiated.

1. Create an archetype project
2. Create a document type 'test'. Commit it
3. Create another document type "test2", select 'test' in the dropdown to choose it as the base type for this document.
4. Commit test2
5. Create a new document

Expected: test2 is available in the list of document types to create
Actual: it is not

I noticed that the prototype of test2 has the hippo:skipindex mixin checked. This prevents it from showing up in the query that builds the create document dropdown list.