We should decouple the logging system from the widget management system

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).

Start by reviewing PR #913 and tracing how the logging system currently depends on widget management. Compare the proposed widget-manager callbacks with the reactivity-system option; done means the two systems no longer depend directly on each other while widget add/remove changes still reach interested logging code.

Written by the indexing model from the issue text.