Allow music with layout instructions in output definitions. (issue 5701049)

[janek . lilypond]

David,

this is 300% awesome!!
However, the following example doesn't work (it doesn't produce any
warning, either):

\score {
  \new Staff { b b }
  \layout { \override NoteHead #'color = #red }
}

Is this intended?  I hoped that it would be possible to do this.
I even hoped that something like this will be possible:

\score { <<
    \new Voice { b b }
    \new Voice = "aa" { b b }
  >>
  \layout { \override aa.NoteHead #'color = #red }
}

but the results are strange: not only does it not compile, but it gives
an unusual error:

GNU LilyPond 2.15.31
Processing `../advanc.ly'
Parsing.../home/janek/lilypond-git/build/out/share/lilypond/current/scm/lily-library.scm:314:40:
In expression (_f "Cannot find context-def \\~a" sym):
/home/janek/lilypond-git/build/out/share/lilypond/current/scm/lily-library.scm:314:40:
Unbound variable: _f

Also, it fails to compile next .ly file in queue (when i tried
processing these files with 2.15.26, advanc.ly failed but other files
were compiled).

HTH,
Janek

http://codereview.appspot.com/5701049/