fix #29406: next/previous handling of grace notes.

The changes here only affect chords with grace notes.

The effect is that next/previous chord commands (right/left arrow) will traverse both grace notes before and grace notes after in the way one might expect - left to right for next-chord, right-to-left for previous-chord, without skipping any or getting hung up anywhere.

If you do this in note input mode, the note input cursor remains on the main note.  See http://musescore.org/en/node/23418.  I'm not sure how much work it would be to get note input mode to work more directly with grace notes, but since it does not work currently, the second commit in this PR actually disables it rather than create the illusion it will work.  That is, if you have a grace note selected upon entering note entry, the cursor will start on the parent note, and left/right keys will skip grace notes while in note input mode.

Eventually of course we could enhance note input mode actually work with grace notes.