Rewrite plugin frontend

[Sebobo]

It's getting harder and harder to extend the plugins frontend part.

As we anyway have to do some rewrites for the backend to support Neos 7
we might also switch to some lightweight frontend library to render
the debug interface.

I would like to try whether we could even use the Neos.Ui React components without
creating huge bundle and this way have a Neos theme.
But as the plugin is anyway only active in development, it shouldn't be a bis issue IMO.