Fix #1427.

Fix #1427.

The copy constructor for Book assumes the list of objects to clone are
scores, which is unsafe since a \book(part) block can also contain markup and
page markers.

* input/regression (book-identifier-markup.ly):

  new regtest

* lily/book.cc (copy constructor):

  treat entries in scores_ separately, cloning Score and Page_marker objects

* lily/include/page-marker.hh, lily/page-marker.cc:

  add copy constructor