debug-layout: Initial draft

First try for new work-flow:

- create private branch `debug-layout-initial` on personal fork
- create pull request against `openlilylib/frescobaldi - debug-layout`

This pull request should be the start for collaborating on the `preview-mode`/`debug-layout` features for Frescobaldi.

It is a complete replacement for the commits in the `preview-mode-experiments` branch, which can be deleted afterwards.

---

- Update command.py: Insert appropriate code to call debug modes
  (currently hard-coded)
- Add include file debug-layout-options.ly
  that manages the checks for more options
- Add modes for coloring voices (only started)
  and displaying control points

Running `Ctrl+M` in Frescobaldi will now compile the score with point-and-click, skylines, control-points and voice-colors.

All files have _not_ been cleaned up in any way yet.