* scm/paper-system.scm (paper-system-annotate): also annotate the

	* scm/paper-system.scm (paper-system-annotate): also annotate the
	estimated pure-Y-extent

	* scm/define-grobs.scm: add pure-Y-extent and pure-Y-offset functions

	* lily/system.cc (break_into_pieces): set the estimated height
	of each child system

	* lily/stem.cc (pure_height): new pure-Y-extent callback

	* lily/staff-symbol-referencer.cc (callback): don't destroy
	the staff-position property

	* lily/hara-kiri-group-spanner.cc (request_suicide): split
	consider_suicide into two functions

	* lily/constrained-breaking.cc (resize): use the new pure
	callbacks to estimate the height of a system

	* lily/axis-group-interface.cc (pure_group_height): new
	side-effect-free VerticalAxisGroup height-callback

	* lily/align-interface.cc (get_extents_aligned_translates):
	split align_elements_to_extents into two functions
	(get_pure_child_y_translation): new function

	* lily/grob.cc: new functions for pure-Y-extent and pure-Y-offset

	* lily/item.cc: new functions pure_is_visible and spanned_rank_iv

	* lily/paper-score.cc: cache break_indices and columns

	* lily/side-position-interface.cc: new pure-Y-extent callbacks