<specDesc> should have option to pull in list of members of a class

[sydb]

Per TEIC/TEI/ issue #1355 “outdated list of declarable elements in chapter CC”, Council thinks that <specList key="model.fooBar"> should have the capability to generate a list of the member elements. A new attribute is in order. Council did not discuss the name, but @include jumps to mind. Council also did not mention attribute classes, but it would make sense if the same happened for key="att.barFoo".