Proposal: support for custom grob and context properties

[Paul]

Hi all,

It would be nice if LilyPond supported user-defined grob and context 
properties, for use in users' code -- in the spirit of practical 
software freedom and to further LilyPond's excellent extensibility. It 
would also be handy for prototyping code that might end up in LilyPond.

Harm recently used custom properties in his BendSpanner-engraver, and I 
use them in my code for alternative notation systems.

Currently this requires finding and copy/pasting the relevant scheme 
procedures so they can be used in your own files.  Namely, 
define-grob-property from scm/define-grob-properties.scm and 
translator-property-description from scm/define-context-properties.scm.

Here are two possibilities for making this easier:

1. Make those two procedures "define-public" so users can just use them 
to define their own properties.  This requires very minimal change to 
LilyPond code, and is convenient for users since existing functions will 
just work for accessing and setting their custom properties (e.g. 
ly:grob-property).

2. Define a "custom-properties" grob property and a "customProperties" 
context property (or whatever name) that each holds an alist of user 
properties.  This adds additional properties to LilyPond and is a little 
more complex for the user to work with, but it provides isolation from 
LilyPond's properties.  (For example, users would only have to worry 
about conflicts with other user-defined properties, not conflicts with 
current or future LilyPond properties.)

I'm interested in implementing this one way or another.  Using the same 
approach for grob and context properties would make sense.

Thoughts?
-Paul