Get rid of the Model class

[oleq]

It used to be a thing until views became observables. ATM the only usage of the Model class is in the heading feature and in a couple of tests related to createListDropdown helper.

I think maintaining this class makes little sense because any kind of observable can handle the task and the scope is very, very limited.

cc @Reinmar, @oskarwrobel

[Reinmar]

I'm removing this from iteration 12. Not sure what it does here.

[oleq]

When working on https://github.com/ckeditor/ckeditor5-ui/issues/262 I found out removing this class is not as easy as I thought. It could be removed but then we would need to do something like

Foo = class extends Observable {}

(or similar) in many places like this one (propagated here) to create an intermediate container for observable attributes, which is super ugly.

If you have any better idea how to do this without Model, I'm open to suggestions.

[Reinmar]

OK, we use it so let's keep it.