BodyCollection UI component

[oskarwrobel]

EditorUIView provides a body collection for child views, detached from the DOM structure of the editor, like balloon panels, etc. Each plugin that introduces such a view uses this collection to display it.

Soon, we gonna have plugins that work out of the editor (Context plugins) and such plugins may need to display a balloon panel somehow. So, we gonna need something like an EditorUIView#body out of the editor.

Let's create a reusable BodyCollection component. EditorUIView will implement it as a body collection, for the editor plugins purpose (like it was before). Context plugin who's gonna needs a body collection will create its own.