preview-mode: Add debug-layout-options.ly base file

Main file that includes the individual debug modes.
Included as -dinclude-settings.
preview-mode: Initial infrastructure in command.py

When composing a command as preview compilation
add several items to it.
preview-mode: Add color-voices mode file

color voices explicitly set
preview-mode: Add display-control-points mode file
preview-mode: Add color-directions mode file
preview-mode: Add display-grob-anchors mode file
preview-mode: Add display-grob-names mode file
preview-mode: Add paper-columns mode file
preview-mode: Add lilypond-version-switch utility

Add a function/predicate 'lilypond-greater-than?' that
can switch the program flow based on the currently
run Lilypond version
preview-mode: Add dockable panel

Basic UI infrastructure for selecting debug modes.

The dockable panel holds check boxes that
switch the different debug options.

When a command is run in preview mode
Frescobaldi checks for these check boxes and composes
the corresponding command line.
preview-mode: Add color-directions mode
preview-mode: Add display-grob-anchors mode
preview-mode: Add display-grob-names mode
preview-mode: Finish color-directions UI
preview-mode: Stub for configuration UI

Configuration variables can be inserted in the document
through the -e commandline option.

This commit shows how to do it and hard-codes anchor-point dots green.

TODO: Complete the function that reads out all values
from the settings.
Try to make sure that only values are added that don't
match the default settings (in order to avoid too long
command lines.

Start Preferences page
-----------------------------
This is really a WIP, just started to show where work has to be done.
Add a new page to the preferences
(which is currently a mere copy of the LilyPond preferences),
add a dummy help page.

Hide work on Preferences UI
------------------------------------
The work on the UI for the configuration Preferences
is postponed to a future release.
Instead of completely reverting the stubs already done
this commit simply hides them so they aren't executed and displayed
preview-mode: Finish debug-layout-options.ly

Small cleanup touches
preview-mode: Add custom-file mode

A debug mode to include a custom file.
This file may contain additional debug modes.

The user is responsible that this file is found
preview-mode: Add paper-columns mode
preview-mode: Reorder checkboxes
add paragraph about module import order