Improve title functions of the wdt module

[peterstadler]

which will currently fail for an input sequence of items

also added/fixed:

• add unit tests for these functions
• exclude non-ab-root diaries from backlinks

[SteffenAstheimer]

Thanks for the PR, especially for including tests. Both the robust functions and the tests look good to me, although there seems to be an issue with the expected results.

I realized one minor issue with the unit tests. I got 12 failures when running the wdt-tests and they all are related to the language. Since expected output is hard coded and the wdt functions don't take language as an argument and just guess it through config:guess-language(), the tests throw failures if the browser is set to a different language than German.
I guess there is no real fix to this other than to keep it in mind when testing.
I saw a similar problem with the quotation marks in the biblio tests, even though each bibl function takes the language as an argument. But some components of the citation are transformed with xsl, which again guesses the language through config:guess-language().
So in other words, I guess there is no easy fix to this problem and unit tests just have to be run on a browser which defaults to german language.

PS: There is one unrelated error (not failure) in the biblio tests for which I'll add a new issue.