Clef support for cue notes

Clef support for cue notes

-) Added \cueDuringWithClef, which allows to specify a clef for
   the cue notes. At the end of the cue section, the clef is
   automatically reset to the containing voice's clef.
-) Cue clefs are implemented as CueClef and CueEndClef grobs,
   created by a dedicated Cue_clef_engraver, which reads
   some cueClef* context properties.
-) After a line break, a cue clef does NOT override the global clef
   of the containing voice, but prints (in smaller size) after
   the containing clef.