Creating start/end cue clefs.

Creating start/end cue clefs.

Create cue clef with cleffedCueDuring
Also added regtest

What's still missing/wrong:
-) Positioning of end cue clefs is messed up (should be before barline,
   not after; should not be repeated in new line)
-) Position of start cue clefs (should be some space between barline and
   clef)
-) Clef glyph for end clef needs to be retrieved from Clef grob (How can
   I do that in the Cue_clef_engraver???)
-) programming error: No spacing entry from Item to `cue-clef'