rfc: basic color support

[Erlend Aasland]

Hello all,

Just for fun, I've hacked up a patch that provides basic color support
(only in the ps backend).

Why color:
Sometimes when I teach music, I find colors to be helpful ("today
we're gonna learn a new note: the f note. All the red notes in this
piece are f's").

Short description of the patch:
scm/define-grob-properties.scm: introduce the color property
scm/output-ps.scm: {set,reset}color commands.
lily/stencil.cc (interpret_stencil_expression): when a "colored
expression" is encountered: save current color, process the
expression, restore color.
lily/system.cc (get_line): check all grobs for color property and
prepare the stencil scheme expressions for further processing in
interpret_stencil_expression().

NB: This patch is just a request for comments. Use it like this:
\override <grob> #'color = #'(1.0 0.0 0.0)
The scheme list is an RGB color. Values must be between 0.0 and 1.0.

Regards,
  Erlend Aasland

color.patch
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