fix #42021: color of drum palette notes

Formerly, Chord::setSelected() used to set all of its notes as well, but that was changed recently.  It's actually a no-op now - it doesn't even mark the chord itself selected.  So I simply select the notes instead of the chord to force the drum palette to display in voice color.

BTW, el->curColor() doesn't really return anything useful here as far as color goes, but we don't need it to - the notes draw themselves in their proper colors.