Fixes RDF export of subobjects

Adds SemanticData from subobjects to the RDF export fixing https://bugzilla.wikimedia.org/show_bug.cgi?id=48708

Note: This is a partial integration of the patch https://gerrit.wikimedia.org/r/#/c/70047/4
Credits to Dbeeson



Code changes in short:
The Export code for RDF now understands that SemanticData has recursive information about subobjects and tries to iterate over them to add to the RDF export.
> I wanted to use a terminator because it is otherwise unclear whether the referenced object is the subobject

If you work with a string then "#" is already used as divider for subobjects ("#" also known as fragment) and when working with a `DIWikiPage` object you can use `getSubobjectName` to determine whether the subject is a subobject or not.

>  I can provide an example if you want.

Would be nice.