Bookmarking labels and page referencing:

Bookmarking labels and page referencing:

 - the \label command can be used at top-level (Page_marker) or inside
   music (LabelEvent);

 - for labels inside music: the paper_column_engraver handles
   label-events and fills the labels property of the paper column;

 - for top-level labels: Paper_book::get_system_specs() fill the labels
   property of the previous prob or of the first column of the previous
   paper_score;

 - when systems are built, the labels of their columns are collected to
   set the system labels property;

 - in Page_breaking::make_pages(), a label->page-number table is built,
   and stored in a variable of the paper_book paper;

 - the \page-ref markup command accesses the table though its layout
   argument. The actual evaluation of the stencil is postponed to the
   layout output phase using a new stencil command:
   delayed-stencil-evaluation;

 - a MARKUP_HEAD_SCM0_MARKUP1_MARKUP2 token has been added to the
   parser for this new markup command.