Propagate super type when appending document type descriptors

Declaring a doctype as extending another doctype while in append mode will discard the super type contributions.

For example, doing:

<doctype name="MyDoc" append="true">
    ...
</doctype>

<doctype name="MyDoc" extends="Folder" append="true">
    ...
</doctype>

Will fail to include the schemas and folders from Folder in B because the superType is not propagated with {[append="true"}}. When merging A with B, if no previous super type is defined for A, the super type of B should prevail.