Issue #768 - chord repetition shortcut: patch for review

[Nicolas Sceaux]

Hi,

Here is patch implementing the chord repetition shortcut that has been
discussed a few times, for review:

  <http://codereview.appspot.com/154056>

I've chosed arbitrary defaults, which may be changed:
- the shortcut is `q';
- the function copying the previous chord only copies the chord pitches,
and removes all other decorations.
Both are customizable, but a sensible default from the beginning may be
preferable.

Nicolas