Move the lyx::biblio namespace into src/. Also make changes to how the BibTeX

Move the lyx::biblio namespace into src/. Also make changes to how the BibTeX
information is acquired. This involves (i) moving the bulk of fillWithBibKeys()
into Biblio.cpp, (ii) introducing a virtual method Inset::fillWithBibKeys() that,
by default, does nothing, and (iii) over-riding this method in the insets that do
need to do something.

This is all preparatory for a more substantial re-working of the BibTex code,
which should follow shortly.

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@19565 a592a061-630c-0410-9148-cb99ea01b6c8