[PATCH] Nested properties in the \paper block

[Joe Neeman]

http://codereview.appspot.com/182042/show

This patch implements nested property setting for \paper block (and
\header block, etc) variables, similar to how \override works for nested
properties. In particular, you can do something like

\paper {
  between-system-spacing #'space = #10
}

to override the 'space sub-property of between-system-spacing without
modifying the other sub-properties. Apart from being more convenient, it
will allow me to write better convert-ly rules for vertical spacing
variables.

Joe