Update code and configuration of the MixinPlugin

A few versions back the wicket plugins for fields in the document editor have been updated. One change was that for these plugins to work a lot less configuration is needed. It seems that the MixinPlugin has been overlooked when this upgrade was done.

It is still working in the project of a client, but with the "extra" configuration. Also: this plugin saves directly to the document node. That should only be done when the Save button is clicked, not by the (de)selection of the mixin checkbox.