"debugging-layout" updated

This pull request replaces the old one about `preview-mode` and incorporates
the discussion of this pull request
(which unfortunately got lost after deleting the corresponding branch).

I deleted the `preview-mode` branch and created a new one.
I updated the contained files and renamed all references to the new suggested conventions.

I think the `debugging-layout` directory in the `snippets` repo will mainly serve
as a discussion place and maybe for some example files, while the actual development
will take place in Frescobaldi and LilyPond repositories.
Therefore I suggest to merge this request and expose the new directory that way.