Issue 1650: merge multiple header specifications. (issue 6445053)

[graham]

LGTM, and I really like the comments in the regtests.  In a few
instances they were slightly unclear, though.


http://codereview.appspot.com/6445053/diff/2001/input/regression/header-book-multiple.ly
File input/regression/header-book-multiple.ly (right):

http://codereview.appspot.com/6445053/diff/2001/input/regression/header-book-multiple.ly#newcode14
input/regression/header-book-multiple.ly:14: % This should NOT replace,
but amend the header:
I spent 30 seconds trying to figure out why you wanted the title to be
printed as
  "Title New Title"
before I realized I was misreading the comment.

Could the comment instead be:
  % This should not replace the entire header, but only replace the
'title' field of the previous header.  The subtitle should be unchanged.


similar change below.  Alternately, you could change the field names:
  \header{ title="don't print this title" subtitle="print subtitle" }
  \header{ title="print this title"}
... etc.  (with proper line-breaks because humans like using \n as a
separator even if lilypond doesn't distinguish between types of
whitespace)

http://codereview.appspot.com/6445053/diff/2001/input/regression/header-book-multiplescores.ly
File input/regression/header-book-multiplescores.ly (right):

http://codereview.appspot.com/6445053/diff/2001/input/regression/header-book-multiplescores.ly#newcode23
input/regression/header-book-multiplescores.ly:23: %% Do we have a
title, and is "New Subtitle" the subtitle?
please change to:
  % this should print "new subtitle".
rather than having a question.

http://codereview.appspot.com/6445053/