Improve Preview Infrastructure

Now (still 'drafting') Frescobaldi's preview command
inserts -dpreview... command line options in
the command string.
frescobaldi-preview-extra.ily checks for the existence of
these command lines and conditionally includes
the corresponding style files.

The options to be included should be made configurable through
Preferences,
and of course the style files themselves are more than
rough so far.

Probably it would be even better to have a small dockable panel with check boxes to check which options should be active (because I assume this isn't a question of general taste but of current task)