Breakable markups with \markuplines.

Breakable markups with \markuplines.

 - \markuplines is a new keyword for entering text that is not
   interpreted as a single markup of several lines, but as several
   one-line markups. The parser is modified to allow this, and
   define-(builtin-)markup-list-command macros are defined.

 - texts are collected into books as list of props. When
   Paper_book::get_system_specs finds a list of more than one props (ie
   a markup list, a list of a single prop being the result of a toplevel
   \markup), it sets their next-space and next-padding properties to
   zero, so that the lines of a paragraph are not stretched on a
   page