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)