shorty: Redesign of the app internal plugin interface

[arkascha]

Shorty offers a plugin interface currently only used by the Tracking plugin.

Future plugins should not simply extend that interface by adding new variants. Instead the interface should be reimplemented in a more object centric way:

• Only one request to each plugin that registers all features and requirements the plugin addresses. This should make debugging and understanding much easier.
• A list of objects derived from a common base class instead of the current list of associative arrays. That would simplify error checking at runtime and at implementation level.