MixinPlugin: better support for setting caption and mixin labels

In the document editor, the MixinPlugin shows a list of mixins that can be removed/added.

The MixinPlugin does not support setting the caption on document type level (it is fixed at 'Metadata' from properties files).

A client on 10.2 words it like this:

Issue 1 is about having different labels for the same mixin when used in different documenttypes
Issue 2 is about to have the functionality of Issue 1 but then for each mixin. Now you can only use one value to show for one mixin in all documentypes. You now have only the wicket:id="mixins" that can be used.

If we start to use {{FieldPluginHelper#getCaptionModel }}in the plugin it will be aligned with NodeFieldPlugin and PropertyFieldPlugin.and the first issue is solved.

Not sure if we can do the second issue: I don't think we should configure the mixin labels on document editor level.

Also we have to decide if we're going to back port this to 10.2 no to 11.2 is enough: client will upgrade