restrict number of parallel running LilyPond instances

[nittka]

On my machine a clean build of a project with many files freezes the system because many LilyPond instances are running simultaneously. I propose that the user can configure the number of parallel executions (on the already existing compiler preference page).

The implementation makes use of scheduling rules preventing multiple jobs with the same rule running at the same time.

[nittka]

If you like the modification in general, please do not merge yet. I'd like to get rid of the static map holding the scheduling rules first. I have an idea and will give it a try, when I have time.

[thSoft]

Isn't MultiRule appropriate for this purpose?

[nittka]

MultiRule sounds more like applying more than one scheduling rule to the same job. What I want is n distinct scheduling rules (where n is the number of parallel executions), that do not conflict with each other but only each with itself.

[thSoft]

Works great, thanks!