Clean up translated docs generation

Clean up translated docs generation

* sort makefile stuff into approriate make/doclang-{vars,rules,targets}.make
* make lilypond-book look for snippets already compiled in directories
given with -I option
* use -I flags for the whole lilypond-book/makeinfo/texi2pdf chain, which
makes building faster than before
* as a consequence, get rid of symlinks from Documentation/user/out-www to
out-www
* add support for big-page HTML manuals (not yet used)

* also update deprecated Python code in lilypond-book (string functions
and string exceptions)