Add dedicated draft-mode directory

This project will surely have a number of files, so I think it is a good idea to work on it within a directory.

I'm not sure if the 'snippets' repo is the right place for that, but as long as we don't have a better solution I suggest to go that way.
Update draft-mode/README.md

Small touches and Markdown corrections.
Add reference to lilypond-implementation.md
Add draft-mode/lilypond-implementation.md

Ideas about how to implement a draft mode in LilyPond
Add sketch for the core 'draft-mode' file
rename draft-mode dir to debugging-layout
debug-layout: Update README.md

Adapt to the new naming
create reference to a (so far empty) frescobaldi document
debug-layout: Update lilypond implementation ideas

Review md document
Update naming conventions