Allow header/footer snippets in test cases, expand lilypond-book to insert versionnumber

Allow header/footer snippets in test cases, expand lilypond-book to insert versionnumber

-) Add --introduction=.. and --footer=.. options to lys-to-tely.py so that we
   can insert custom text form a texi file at the beginning and end of a
   regression test page. So far, one had to add a .ly file with a proper name
   (so that it gets sorted to the begin/end), which resulted in an ugly empty
   image and a link to a dummy .ly file at the top. The header for the
	 lys-to-tely script must not be called *.texi, because make WWW tries to
	 run makeinfo on all *.texi files.
-) Change the buildsystem to use these additional parameters. The make variables
   HEADER_FILE and FOOTER_FILE are used for this.
-) Change the regression tests to make use of the texi file rather than the
   AAA-intro-regression.ly file
-) Since that header contained the lilypond version number, I also expanded
   lilypond-book to replace @lilypondversion (or the \lilypondversion and
   <lilypondversion> counterparts for latex and html) by the version. The
   build must then use the lilypond-book script from scripts/out/ rather
   than scripts/lilypond-book.py (which does not have the @VERSION_NUMBER@
   replaced by the version by the build system).