Issue 3673: Make make-relative able to deal with music rather than just pitches

Issue 3673: Make make-relative able to deal with music rather than just pitches

The old behavior of make-relative was too cumbersome to use in many
cases.

This variant allows, for example:

withOctave =
  (ly:music?)
  (make-relative
   (music) music
   #{ \context Bottom << $music \transpose c c' $music >> #}))

\relative
\new Staff \withOctave {
  \partial 4. c'8 e g |
  c2 e,4 g |
  c,8 c' b a <g d'> <f c'> <e b'> <d a'> |
  <c g'>1 | \bar "|."
}