Add basic SwiftPM support for Objective-C code

[Jeehut]

This fixes #158 for the most part. Only the Swift code adding the ObservableObject support I couldn't include because SwiftPM didn't want to accept mixed language packages. Maybe that can be fixed by splitting to two targets somehow, but as we don't need it for our project, I didn't invest the time to check. But this should be better than nothing.