Add a new kind of object, Page_marker, at the same level as scores and

Add a new kind of object, Page_marker, at the same level as scores and
markups, holding page breaking and page turning information, and which
can be used to create top-level page breaking and page turning
directives.  Thus, \pageBreak, \noPageBreak, etc, can be used at
top-level, between scores and top-level markups.