* Documentation/user/refman.itely (Gregorian square neumes

	* Documentation/user/refman.itely (Gregorian square neumes
	ligatures): fixed typo

	* lily/vaticana-ligature-engraver.cc,
	lily/include/vaticana-ligature.hh: bugfix: use special heads for
	lower/upper head of Pes only when heads are stacked

	* lily/vaticana-ligature-engraver.cc, lily/vaticana-ligature.cc,
	scm/define-grob-properties.scm: cleanup: make "join-left" a
	boolean grob property and save delta pitch in grob property
	"delta-pitch" instead

	* lily/include/gregorian-ligature.hh: bugfix: corrected comments