Merge pull request #523 from ckeditor/t/ckeditor5/416d

Feature: Implemented automatic items grouping in the `ToolbarView` component. Closes ckeditor/ckeditor5#416.

BREAKING CHANGE: The internal structure of the component has changed. Toolbar items are no longer direct descendants of the toolbar in DOM, which may affect some integrations (mainly CSS selectors if adjustments were made to the styles).

Author: jodator