Add "Scale vertical spacing"

Provides an easier way to adjust vertical spacing, letting you scale the default settings, see:
http://lilypond.org/doc/v2.18/Documentation/notation/flexible-vertical-spacing-paper-variables
http://lilypond.org/doc/v2.18/Documentation/notation/flexible-vertical-spacing-within-systems
My idea was that since the function takes such a large number of options, it would be easier to cut and paste from an example of the full default input, rather than trying to remember everything.  And so having that in the file makes that easier to do.  For example, you could include the file, easily open it with command-click in Frescobaldi, and then cut and paste the input example and adjust it as needed.  It seems like it would not be as easy to access that in the example file.

But, all of that said, removing this is ok with me, if you think that's better and more consistent for the library.