Give more flexibility to subclasses of EvidenceSource with panel init

When creating a subclass of EvidenceSource that implements several rules in it and each rule needs its own ("non-uniform") panel, the current interface does not allow to do it easy enough. (I need that flexibility now, indeed, for one of my implementations; work in progress.)

I propose to add initWithPanel: to class EvidenceSource. And also a ready class method for loading panels from nib files. With that addition, I will be happier as a developer of evidence sources...