We should decouple the logging system from the widget management system

[rafaellehmkuhl]

On PR #913 we added a coupling between the logging system and the widget management system.
We did that as it was a quicker solution, so we don't hurt the stable roadmap, as there are other PRs waiting for this one.

We should try to decouple them again.

One of the ideas to solve this is to allow register of add/remove callbacks on the widget manager, so those interested can react to changes there. This can also be accomplished through the reactivity system (proxys).