Make <> a better supported citizen.

Consists of two commits each touching one file:

Let display-music work more consistently with command events and empty chords

Make chord repeats q ignore <>

http://codereview.appspot.com/6191052

Patchy the autobot says: LGTM. but with warning: 1 line adds whitespace errors.

Indeed. Fixed the whitespace error locally; does not warrant a new upload since I'll be pushing the patch eventually myself.

Make <> a better supported citizen.

Consists of two commits each touching one file:

Let display-music work more consistently with command events and empty chords

Make chord repeats q ignore <>

http://codereview.appspot.com/6191052

While the related discussion http://lists.gnu.org/archive/html/lilypond-devel/2012-05/msg00093.html> could not be brought to bother about the preferable print form of empty chords and rather focused on the need to hide the functionality from users, for consistency's sake I decided to print them as < > rather than <> since is printed as < c > as well. Since the only change to the previously tested state has been a one-character change in the respective print string (and it is not exercised in the regtests), I am resetting the status to Patch-review manually.

The above-mentioned discussion is actually about issue 2522, namely documenting the behavior of <> and, most controversely, making use of it in examples. This patch, in contrast, is only about making it behave consistently in some less common situations.

Since I currently have a severe patch buildup of overlapping patches in define-music-display-methods, some of them unfortunately covering critical regressions, I am pushing this early as fea33b81da974ff0627843dfc88a00b49cd2323c