To avoid duplicity, remove natbib_authoryear and natbib_numerical
and replace them by natbib, and keep track of the engine `type'
in the new \cite_engine_type document setting. This will make it
easier to add more citation engines.
LyX format incremented to 424.
git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@40592 a592a061-630c-0410-9148-cb99ea01b6c8
