Fix Info documentation with images

Fix Info documentation with images

This patch mainly applies suggestions from David Kastrup and Jan.

* fix symlinks creation from prefix/share/info to
prefix/share/doc/lilypond, enable it even when installing in
nonstandard prefix, and clean it up in the makefiles,

* add info and info-install toplevel targets,

* improve documentation (make help, stdout prints and Application
  Usage),

* fix flats graphics in Introduction for Info format.
(cherry picked from commit 0c9af42587ceac7e72290435164f6a9394250c66)