Preview Mode Enhancements

This pull request offers a set of 'debug modes' for Frescobaldi's Preview Mode.
A dockable panel contains checkboxes to select modes that will be active when Frescobaldi compiles a file in preview mode.
Currently implemented:

- Display control points
- Color explicit voices
- Color explicit directions
- Display grob anchors
- Display grob names
- Display skylines
- Display paper column info

The programming and the infrastructure is by me, but the individual modes are (partially heavily) based on work by David Nalesnik, Thomas Morley, David Kastrup and Janek Warchoł, as attributed in the individual mode files in the `preview_mode/` directory.

**To be done (later):**

- A mode to visualize offsets (postponed?)
- Preferences page to configure the appearance of the modes (postponed)
- Where can I add documentation?
- Save the preview PDF with a filename suffix

See also https://github.com/openlilylib/frescobaldi/wiki/Preview-Mode-Development-Roadmap