GroupRelationType plugins now have ui_text_provider handlers, GCT entities lose label/description properties

You can now specify a ui_text_provider handler for your plugins to affect how a plugin's corresponding GroupContent entities and GroupContentType entity are presented in the UI. This will allow you to change labels and descriptions on a per-plugin basis.

As a result, the GroupContentType entity no longer has a label or description property of its own. The GroupContent entity now calls the handler for its label too.