Add support for custom CSS classes in the format plugin

The format plugin currently does not provide the capability to assign custom CSS-classes to blocks in the Multisplit-Frame of the Editor Toolbar. This commit enables a user to add another "availableButton" (e.g. p.myCustomCssClass) to the Toolbar. The new Item in the toolbar will then be available via a dynamically rendered Button that displays the Tag-Type and the Class-Name.