Chord repetition: \relative mode, cautionary accidentals

Chord repetition: \relative mode, cautionary accidentals

\relative mode: Introduce a new RepeatedChord music type, which element
property is the copied chord (as produced by the chord repetition function).
Define a relative callback for repeated chords, which avoids
octaviation of repeated chords, by modifying the first note octave
before applying the octave relativization.

Only memorize <..> chords.

When copying a chord, unset cautionary and forced accidental (fix #925).