Fix #786.

Fix #786.

Send a CompletizeExtenderEvent at the end of each lyrics block so that
the Extender_engraver can tie up loose ends with any pending extender.

* input/regression (display-lily-tests.ly):

  add extra space to \lyricsto and \addlyrics test to account for silenced
  display method

* lily/extender-engraver.cc:

  add listener for CompletizeExtenderEvent which will call completize_extender ()
  if necessary

  revert previous fix which checked for empty LyricText grob

* lily/hyphen-engraver.cc:

  revert fix as above

* lily/lyric-engraver.cc (process_music):

  don't create empty LyricText grobs

* scm/define-event-classes.scm (event-classes):

  add completize-extender-event as child of music-event

* scm/define-music-display-methods.scm:

  add display method for CompletizeExtenderEvent (returns empty string)

* scm/define-music-types.scm (music-descriptions):

  add CompletizeExtenderEvent

* scm/ly-syntax-constructors.scm (lyric-combine-music):

  insert CompletizeExtenderEvent at the end of the sequential music (i.e.,
  following the last lyric)