Move langdefs.py to python, and clean up doc building scripts

Move langdefs.py to python, and clean up doc building scripts

* move langdefs.py to python/, so that lilypond-book and
scripts in buildscripts can load it more easily

* move lilypond-doc gettext domain loading to langdefs.py
(this avoids code duplication in a lot of doc building scripts)

* match docs building with new lp-boo naming XX/lily-XXXXXXXX
(this fixes a lot of broken ly images and lily-*.ly links)