BibKnowledge: lxml port get_kbt_items_for_bibedit

* Replaces usage of `libxml2` and `libxslt` with `lxml` in
  `get_kbt_items_for_bibedit` function.

Signed-off-by: Jiri Kuncar <jiri.kuncar@cern.ch>
BibKnowledge: fix get_kbt_items_for_bibedit

* Uses either libxml2 or lxml depending on which library is present.
  (closes #1879) (closes #1895)

Signed-off-by: Yoan Blanc <yoan.blanc@cern.ch>
Tested-by: Tibor Simko <tibor.simko@cern.ch>
installation: lxml recommended

* Recommends lxml now that libxml2 and libxslt1 are fully optional.

Signed-off-by: Tibor Simko <tibor.simko@cern.ch>