Fix #23371 : TAB - grace chords fretted incorrectly

Fix #23371 : in TAB's grace chords are not fretted at all or fretted incorrectly.

Since the grace chords have been moved from a parent segment of their own to a parent chord, the fretting algorithm no longer 'sees' them correctly.

Fixed by extending the fretting algorithm to deal with chords not belonging to segments.