Declaring (Java) Classes

[Stefterv]

In my work on the Gradle plugin I'm scanning the jars within libraries for classes that are being used by the potential sketch.

I think it would be helpful to have another field on the contributions, that declares what classes the library contains, we could use this at a later stage for auto-importing libraries as they are being used.

I think it would be helpful if these classes are user-defined as the jars contain a lot of classes that are not meant to be directly imported

as an example, let's say that controlp5 has declared that it should be imported for controlP5.* classes, then when the user opens a sketch from the web that imports p5, we can pop up a helpful dialog inviting the user to download the necessary library

[mingness]

I see the sense of this, and it takes care of processing/processing-library-template#78. If it's user-defined, maybe we want to help the developer to test that this configuration is correct - none of the other library properties impact installation the way this does.