* lily/coherent-ligature-engraver.cc,

* lily/coherent-ligature-engraver.cc,
lily/gregorian-ligature-engraver.cc,
lily/ligature-bracket-engraver.cc, lily/ligature-engraver.cc,
lily/mensural-ligature-engraver.cc,
lily/vaticana-ligature-engraver.cc,
lily/include/gregorian-ligature-engraver.hh: updated for new
Coherent_ligature_engraver; added comments that describe the basic
design ideas of the ligature implementation

* lily/coherent-ligature-engraver.cc,
lily/include/coherent-ligature-engraver.hh: new file: shared code
between mensural ligatures and Gregorian chant notation ligatures