Issue 1650: Multiple header blocks shall be merged rather than replace a previous one (issue 6441058)

[dak]

http://codereview.appspot.com/6441058/diff/1/lily/parser.yy
File lily/parser.yy (right):

http://codereview.appspot.com/6441058/diff/1/lily/parser.yy#newcode932
lily/parser.yy:932: /* Copy the new header fields, i.e. overwrite
existing, but preserve unchanged ones */
I think this is the wrong way to do things since you can't refer to old
header values when defining the new one.  Instead, you should start with
the definition you are adding things to, like LAYOUT and similar do.  So
that likely means that you need to change the definition of
lilypond_header (or create several versions of it) to make it not start
from an empty slate.

http://codereview.appspot.com/6441058/