Add preview mode material to Frescobaldi

**Preliminary commit** just to show an idea.

(You can't use it right away because of the hard-coded path in `command.py`)

I suggest storing preview material as separate include files in
a Frescobaldi subdirectory and optionally
include them as single features.

In the preferences there one can select if one wants the
preview behaviour at all and can then select which features
to be used:
- control-points
- coloring of voices (should be improved)
- coloring of tweaks
- etc.