Selectable variants configurable in document type editor

The imagepicker in the richtext editor has an option to select the variants of an image.

Desired is to configure this in the document type editor and document this.

mdenburger]
Review done, noted some minor code comments in https://source.onehippo.com/cru/CMS7-135.

The two new configuration parameters (included.image.variants and excluded.image.variants) should also be added to the default configuration of the hippostd:html template [1]. Please also add them to the 'default' parameters, prefixed with 'imagepicker.' (see 'preferred.image.variant' for an example). This way you can easily edit these parameters in the document type editor. Please also describe the two new configuration parameters in the CKEditor documentation at Hippo Campus [2].

Once you've processed the feedback you can mark this issue as resolved so QA can test it.

[1] in CMS trunk: richtext/repository/src/main/resources/hippostd-html-template.xml
[2] https://cms.onehippo.org/?path=/content/documents/hippo-cms-7.9/library/concepts/document-types/configure-the-html-fields