I couldn't hold myself and renamed a bunch of stuff making this way to big.
Will undo the renaming and then move it out of draft.

Maybe we should get that before 0.7.0.

Stuff I am not that happy with yet:

• registerSelf can't be virtual, but we need m_self to safely create a ref to the current widget.
• using lambdas for the callbacks kind of makes the static handler methods in the widgets redundant. Perhaps removing them and just calling the widgets methods in the lambda directly is better.