* 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.