Add a Buffer::fully_loaded member function, returning true only when

Add a Buffer::fully_loaded member function, returning true only when
the file has been loaded fully. Used to prevent the premature generation
of previews and by the citation inset to prevent computation of
the natbib-style label.

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@7953 a592a061-630c-0410-9148-cb99ea01b6c8