Move label generation from InsetBibtex to BiblioInfo.

Move label generation from InsetBibtex to BiblioInfo.

This only move the code, but does not change the displayed labels.
Thus for numerical citation, the label is set to the cite number;
for author-year citation, the abbreviated list of authors is used
i.e. "Smith et al. 2001".

Eventually, we might want to make the label customizable, or get
it from BibTeX.