[PATCH] Fix key signatures with cue clefs

[Reinhold Kainhofer]

CueClefs: Use middleCClefPosition in the key engraver

Otherwise the key signature would use the middle staff position of the cue part
(so the keysignature would be printed as if it were in the cue clef rather 
than the real clef), which is not what we want.

Please review:
http://codereview.appspot.com/4551101

Cheers,
Reinhold
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Reinhold Kainhofer, Vienna University of Technology, Austria
email: address@hidden, http://reinhold.kainhofer.com/
 * Financial and Actuarial Mathematics, TU Wien, http://www.fam.tuwien.ac.at/
 * Edition Kainhofer Music Publishing, http://www.edition-kainhofer.com/
 * LilyPond music typesetting software, http://www.lilypond.org/