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.
@mkroetzsch 
Do you know the rationale behind using '#-'?
If you can approve that I can document it somewhere and we can move ahead.
'#' is also being used at other places in the code now (see below getLongCaptionText() and getLongCaptionText())