Let \relative work on \transpose content as if it were untransposed

Let \relative work on \transpose content as if it were untransposed

In that manner, injecting a \transpose inside of \relative music will
not change the resulting meaning apart from transposing the relative
passage in question by the specified amount.


This is mainly some supplemental discussion material for issue 4366:
actually committing it would require serious discussion on user and
developer list and looking at possible effects on existing scores.
Possibly conversion rules.

However, I find in the CG GLISS chapter the question "Why doesn't
\transpose respect \relative mode?", there is

    Known issues and warnings
    .........................

    The relative conversion will not affect ‘\transpose’, ‘\chordmode’ or
    ‘\relative’ sections in its argument.  To use relative mode within
    transposed music, an additional ‘\relative’ must be placed inside
    ‘\transpose’.

and it is also quite infrequently a user mailing list question.  Would
users likely consider this behavior "more correct" than "unexpected"
absolute mode?


Also contains commit:

Let \displayLilyMusic handle \transpose properly

Not sure this one is actually a good idea: people might _want_ to see
the transposed material rather than its input.