Introduce viewDocument#layoutChanged event.

[oskarwrobel]

Now we use view#render event to know that editor layout has changed. The UI uses this event to keep floating stuff properly displayed after all view changes. The problem is that render event is fired and should only be fired along with the view rendering process but unfortunately rendering is not the only way to change the layout. E.g. layout can change as a result of resizing

or when the source of the <img/> element is loaded async https://github.com/ckeditor/ckeditor5-image/issues/142 and it can be more and more reasons.

We decided to introduce viewDocument#layoutChanged event. This event should be fired after each view#render (we can consider to reduce the event frequency using throttle function), besides can be fired every time when lyout has changed other way than the view rendering.