preview-mode: display-control-points Add various elements

- Make cross- and line-color configurable (independently)
  (and let it accept _any_ color definition instead of only a RGB list)
- Enable it for _all_ curve types
- Prepare code for printing also the first/last control-point
  and the line between CP 2-3
- Solve the Tie issue for 2.16.2
- Provide a workaround for Tie issue in 2.17
  (TODO: How to integrate this properly?)

(TODO optionally: Make printing of CP 1/4 or line 2-3 configurable)
preview-mode: Rename control-points- vars to -debug-control-points-

When implemented in LilyPond itself we should have a 'namespace'
to avoid dublicate variable names.
All our configuration variables should be prefixed 'debug-'
preview-mode: make grob-anchor's color configurable
preview-mode: Make grob-types (for display-anchors) configurable

Todo: This should definitely be configurable in the Frescobaldi UI
(within the panel) because 'all-grobs is usually too intrusive.

So we need to figure out how to define variables on the command line.
preview-mode: Rename printRefpoint to printAnchors
preview-mode: Move voice-color configuration to color-voices.ily

It seems the configuration options (variables) should be
located in the respective files, not in the main include file.
preview-mode: Refactor \voiceXXX redefinitions
preview-mode: Move direction-color to color-directions.ly

Clean up and extend this file (still unfinished)

TODO:
- catch objects that are directed with ^ or _ operators
- add more object types
preview-mode: Add loading code for LilyPond

Add (commented) code that will be run when we implement this
in LilyPond.
Add more explanation in comments.