Frescobaldi testing requested

[Urs Liska]

Hi to all those who use Frescobaldi from the Git repository,

I've done some refactoring of the code handling LilyPond jobs, as a 
preparation for further work. So far there *should* be no differences in 
behaviour or performance, but I think my code introduces plenty 
opportunity for new bugs and unwanted side-effects. Therefore I'd be 
glad about anyone who tests https://github.com/wbsoft/frescobaldi/pull/1100.

As there are no new functions involved it's "only" about stability and 
errors, with anything that involves external processes like LilyPond, 
the regular engrave modes, the score wizard preview etc. (the only thing 
I've deliberately not fixed *now* is the "show available fonts" dialog).

Thanks
Urs