* lily/ligature-engaver.cc, lily/coherent-ligature-engaver.cc,

	* lily/ligature-engaver.cc, lily/coherent-ligature-engaver.cc,
	lily/gregorian-ligature-engaver.cc,
	lily/include/ligature-engraver.hh,
	lily/include/coherent-ligature-engraver.hh,
	lily/include/gregorian-ligature-engraver.hh: make these classes
	truely abstract.  This should fix some internal oddities such as
	duplicate as well as dead translator/property declarations, and
	also some dead code.

	* lily/ligature-bracket-engraver.cc, lily/ligature-engraver.cc:
	bugfix: avoid message "junking empty ligature" on ligature
	brackets by collecting dummy grobs.