Implement ObservableMixin.decorate()

[Reinmar]

A common pattern which we use is that in order to make some component's action overridable we implement it using an event. The general flow is like this:

1. The method does only one thing – fires an event (called like that method).
2. The default action of that method is implemented in form of a listener with a default priority.

One can then easily listen to that event, modify arguments, cancel it, perform additional actions, etc.

We always did this manually which means adding a bit of ugly code and is not DRY. When talking about https://github.com/ckeditor/ckeditor5-core/issues/88 we thought that execute() could be implemented in the same way but we can't have even more code duplication and the commands' code should be as clean as possible.

Hence, the idea to implement OM.decorate() (big simplification of the decorators idea). It will take care of replacing the original method with a method which fires and event and plugs a listener.

EventInfo#return

Part two – implement EventInfo#return so you can return from fire(). This will be needed for https://github.com/ckeditor/ckeditor5-engine/issues/963 and it makes no sense to work on these two things separately.