Create different pdf layers

[Urs Liska]

I have brought this up before without success. But as my interest 
reappeared I'll try it again with a slightly more specific question.

When LilyPond finally renders its objects 'on paper' how complicated 
would it be to allow it to print on layers that show up as separate 
layers in the final pdf?
I would consider this a very useful enhancement.

As a first step this could and should be done without any layout 
considerations, i.e. without changing anything in the layout engine. 
Simply put grobs on the default or a dedicated layer with a syntax 
something like

\new Layer = "Annotations"
\change Layer = "Annotations"
\change Layer = "Default"

Instead of the \new Layer command I also could imagine defining layers 
in the \paper block

There is much more potential in this, but just to show what I mean (with 
a known construct):
If I could print the result of annotate-spacing on a different layer I 
could simply switch that layer on and off in a pdf viewer.
Or if I have layout-indifferent additions like the control-points 
visualization in
http://lilypondblog.org/wp-content/uploads/2013/07/chopin-beams-5.preview.png
it would be nice to be able to switch them on and off without having to 
recompile the file.

Any ideas?

Urs

PS: As a first step it already would be nice to know
- where (in the code) LilyPond actually 'prints' its objects (and where to)
- where I can find concise and understandable information about how PDF 
layers are created
  (in the sense of creating them when writing a file, not how to create 
them in Acrobat or the like)