* input/regression/optimal-page-breaking-hstretch.ly: test for

	* input/regression/optimal-page-breaking-hstretch.ly: test for
	ragged-last-bottom also

	* lily/paper-column-engraver.cc (finalize): make the end of a score
	breakable by default. This is to balance out a change in behaviour
	of the page-turn-breaker which no longer makes the end of a score
	breakable.

	* lily/paper-book.cc (pages): set the systems_ once the pages are
	broken

	* lily/page-turn-page-breaking.cc (calc_subproblem): use the new
	Page_breaking interface.

	* lily/page-breaking.cc (class Page_breaking): make the interface
	more consistent and provide abstractions for dealing with
	Line_divisions.

	* lily/optimal-page-breaking.cc (solve): use a more straightforward
	algorithm. Use the new interface to Page_breaking.

	* lily/page-spacing.cc: better support for ragged-bottom and
	ragged-last-bottom