* lily/paper-outputter.cc (output_stencil): dump font definitions

* lily/paper-outputter.cc (output_stencil): dump font definitions
before each stencil.

* lily/include/paper-book.hh (struct Score_lines): new
struct. Collect info per Paper-score.

* lily/include/page.hh (class Page): to_stencil() returns Stencil
everywhere.

* lily/stencil.cc (find_expression_fonts): new function

* lily/paper-outputter.cc (output_stencil): use
interpret_stencil_expr

* lily/stencil.cc (LY_DEFINE): ly_stencil_fonts: new function.
(interpret_stencil_expr): new function. Generic stencil
interpretation.