Fix #435: refactor intra-xref code

[tomschr]

Some xrefs were not detected as intra-xrefs.

• Generalize code of how intra-xrefs are detected
• Make code consistent between HTML & FO
• Correct wrong @lang -> @xml:lang due to DocBook5

TODOs:

• [x] test it for HTML
• [x] test it for FO

[tomschr]

Okay... I've cleaned up the code and did some tests in HTML & FO:

Tests with normal and intra-xref

Using this code inside `art_amd-sev.xml`: ```xml See . (xref pointing to a table in this article) See . (xref pointing into another chapter outside of this article) See . (xref directly to a book) See . (xref directly to an article) See . (xref pointing into another chapter outside of this article, but with xrefstyle) See (xref directly to an article, but with xrefstyle) ``` with this result: 

The result is the same for HTML & FO. I still fear that I forget a test case. Tomas also tested it in the meantime and he didn't find anything.

@sknorr If you give your final okay, we could finish this PR soon. Are all of your issues resolved? Anything else you would like to see? Hope the indentation is correct now. :innocent:

Thanks!

[tomschr]

Many thanks Stefan!