Rewire lilypond-book output.

Rewire lilypond-book output.

For lilypond-book, toplevel elements (markup, score, music) are now
collected in a single \book, which is then output system for system.
This has several consequences

- toplevel markups are now output without taking up a complete page

- multiple scores in a .ly file all end up in the output

- naming of output files has been changed.  Multiple scores don't
result in base-1-1, base-2-1, .. naming.

- In LP-book output toplevel \layout{} and \paper {} settings
influences appearance of preceding scores.