Add custom selectable css classes

Please read this before updating from dieschittigs/contao-dieschittigs-helpers-bundle!

Now the classes will be stored in the database. The classes set in tl_settings will not be used anymore. There is no backwards-compatibility, so you have to set all classes again, even in the pages, articles and elements!

classes-bundle

• Adds functions to manage and add classes for css styling in the contao backend
• Add classes in your theme for the use in pages, articles and content elements