Review our approach to MVVM

[Reinmar]

Currently, we've implemented MVC or MV(C)VM, depending on a component. There's been ongoing discussion between us on which approach is better and we couldn't make a final decision, so the code isn't very consistent at the moment.

We need to verify which pattern better fit into our lib. So far we think that MV(C)VM will do best, however, we need to improve the initialisation of such components.

Currently, there's an issue with how view model can be created by a controller (and then passed to view), while controller should also be given its view in its constructor. This means initialisation like this:

const model = new Model();
const controller = new Controller( model );
const view = new View( controller.viewModel );

controller.view = view;

This is obviously ugly. Fortunately, we seem to agree that since view model is very tightly bound to the view, it can be instantiated by that view, to be later retrieved and controlled by the controller:

const model = new Model();
const view = new View(); // it creates view.model
const controller = new Controller( model, view ); // controller reads view.model

[Reinmar]

One of the things that needs to be fixed is: ckeditor/ckeditor5-ui-default#11 (comment)

[Reinmar]

One of the things that would need to be standardized is where we keep docs for the model and view model interfaces. View models can be documented inside views I guess and model perhaps inside controllers.

[Reinmar]

Related issue: https://github.com/ckeditor/ckeditor5-ui/issues/31

[Reinmar]

While refactoring pieces of this code I noticed that many models lack documentation. This needs to be fixed as part of this ticket.

[Reinmar]

Related issue ckeditor/ckeditor5-ui-default#13.