Bugfix SUP-6294
When copying lists which are styled with custom classes (via the list plugin), the custom classes where not preserved on paste. To fix this
issue the generic content handler was rewritten: plugins can now register a PluginContentHandler and are able to define which parts of a
pasted content should be handled by a PluginContentHandler instead of the GenericContentHandler. For the list plugin a PluginContentHandler
was implemented which preserves the configured classes and also adds the configured default classes.