Feature: multiple header definitions

[Graham Percival]

Feature request: keep old \header{} info when a new \header{} is
defined.  In the example below, the "composer" field is lost due
to the \header{ title =""}.

\layout{} and \paper{} can be called multiple times; could \header{}
work the same way?

This would be useful when dealing with multiple pieces in a large
set -- say, define to the composer and a fancy copyright field in
a global-init.ly file.


\version "2.6.0"
\header{
composer = "Composer"
}
\header{
title = "Title"
}
{ c'4 }