Preview-mode: Refactor UI using loops

Don't have to explicitly create and handle each checkbox
individually. Now we have a dictionary of modes that can be
processed through loops
Separate Point-and-click from Preview modes in Engrave Custom
preview-mode: Improve coloring of voices

Now also manual stems benefit from the >=2.17.6 improvement.
They use \temporary\override then, so after reverting the
stem direction to neutral it won't revert to black but to the
previously active color.

As the other colorings (slurs, dynamics) don't interfere
with the coloring of voices I consider this issue fixed
with this commit.
preview-modes: Include lilypond-version-predicates

While this isn't needed currently it is a nicer and more
comprehensive file that may prove useful later
Merge pull request #260 from uliska/preview-mode-refactoring

Engrave Custom: Separate P-A-C from Preview Modes