Establish extension API policy and unit test changes

Once in a while I'm reminded that while we have Since here and there there's no well-defined version concept as far as extensions go, and I'm pretty sure there's small changes now and then nobody noticed.

libpeas, which we'll be adopting, doesn't really handle versioning.

As I've taken this approach elsewhere I would suggest to have a vapi-like syntax file, generated by a script that reads vapi files and checks a previously generated file for changes. The Vala syntax is concise, easy to read and can be generated from existing vapi files. I'm saying -like here because it could be any other format, whichever is easiest to work with. And maybe this even can be contributed to libpeas if it works well.