Check/integrate the smufl module

[uliska]

There's a smufl directory copied over from the snippets repository.

We have to turn that into a proper OLL module and check if it's properly integrated and documented.
I think it will be a good thing to provide at least half-baked SMuFL support in this package.

[uliska]

Following the discussion in #6 I add the following tasks:

• Remove the Bravura font files
• Properly reference download sites for SMuFL and Bravura
• Check if usage of Bravura and arbitrary other SMuFL fonts is documented
• Disentangle the definitions.ily file:
  • Turn it into a proper "openLilyLib module"
  • Separate the SMuFL compatibility layer from the Bravura stylesheet.
    The latter should be loaded separately, as a "component" of the smufl module:
    \loadModule notation-fonts.smufl.bravura