Refactoring cross-ref resolution
The main benefit of this refactoring, besides a general code
modernization, is, hopefully, proper support of BibLaTeX's 'xdata'
field.
The number of crossrefResolvedEntry(..) functions got reduced from two
to one (the static variant did not bring any benefits).
Minor potential premature delete on an Entry object fixed in
FileExporterXML::write(..).