Make moderation/workflow states fieldable

Problem/Motivation

Make moderation/workflow states fieldable. Then it would be more systematic to store attributes of a state at the state object itself.
Once using it in views one could achieve all kinds of useful things:

E.g. color of the font that could be used in views according to the state.
E.g. css background-color code for display.
E.g. Additional Links for further information regarding the state.
E.g. Actions and information concerning the next possible transitions.
...

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet