Allow music in contextmods

Allow music in contextmods
so that one can say \new Staff \with { \clef bass } { ... } or similar.

It turns out that \time ... is rather elusive to get to work in such
circumstances (context mod or output def) since the alias for the
Timing context is established by the Timing_engraver, namely _after_
the context modifications have already been set up.  Apart from that,
most other things work nicely.

Consists of three subpatches:


Use elements-callback for layout-instruction event reinterpretation


Allow music in contextmods


Implement 'apply context mod

[janek, 2012-03-10 21:48:56]

David,

i'm sorry because i cannot give your patch a proper review, but i appreciate
your work.  I think Lily really needs changes like this.

thanks,
Janek