Change Info docs setup and clean up Documentation/user/GNUmakefile

Change Info docs setup and clean up Documentation/user/GNUmakefile

* install Info files in $(infodir) iso. $(package_infodir)

* put Info images in lilypond/ subdirectory (actually symlink
  $(webdir)/Documentation/user/ to lilypond), which required
     - adding lilypond-book option --info-images-dir and makefile
      variable INFO_IMAGES_DIR,
     - adding @sourceimage macro in macros.itexi to include
      images from the source directory,
     - making a symlink lilypond -> out-www so that 'makeinfo --info'
      can find images.

* clean up Documentation/user/GNUmakefile and : use generic rules, use
  --tidy for texi2pdf, symlink PNGs only for Info docs, ...

* update Info directory entry: fix convert-ly entry, add entries for
  all manuals.

Also fix nitpicks in www_post.py:
* don't link lily-*.txt (which only contain "image of music") to
  out-www/*-root

* add redirection page from input/lsr to documentation index.

The makefile clean up was also made necessary because the Music
Glossary wasn't generated in Info format with images in out-www/ with
the previous revision, and I couldn't figure out why.