[Backport 7.9] - Image set uses inconsistent translation mechanism

Essentials provides a plugin to create and configure new (custom) image sets by drawing a copy of /hippo:namespaces/hippogallery/imageset. While trying to customize the variant translations mechanism, I noticed that the translation mechanism provided by hippogallery:imageset does not support translation the image variant captions.

Apparently, there are two separate translation mechanisms in place: (a) one (implemented in hippogallery:imageset) affecting the variant names as shown in the CK Editor image picker variant selection drop-down, and (b) one (not implemented in hippogallery:imageset) affecting the variant names as shown in the CMS image gallery.

(a) makes use of the nodes /hippo:namespaces/hippogallery/imageset/hippo:translation[X], where X is high enough to grab a node with the hippo:property property.
(b) is described roughly in [1].

I suggest to refactor this into using a single translations mechanism (preferably b, I believe more people are familiar with that one, and it is more "readable").
In order to provide a more solid basis for Essentials' gallery manager, hippogallery:imageset should also support b) out of the box.

[1] http://www.onehippo.org/library/concepts/editor-interface/add-internationalization-i18n-to-document-types.html