Add easy retrieval of images in the new imageset

Hippo CMS 7.5 contains a new imageset type, described on the wiki at https://wiki.onehippo.com/display/CMS7/Create+a+custom+image+set. The new image set stores the width and height of each image in the set, and the original file name. The HST should include a way to easily retrieve the images in an image set and their width and height.

The definition of the new image set is located in hippogallery.cnd:

[hippogallery:image] > hippo:resource

• hippogallery:width (long)
• hippogallery:height (long)

[hippogallery:imageset] > hippo:document orderable

• hippogallery:filename (string)
• hippogallery:description (string)
  + hippogallery:thumbnail (hippogallery:image) = hippogallery:image primary mandatory autocreated
  + hippogallery:original (hippogallery:image) = hippogallery:image

Standard beans that map to these types would be good. The tricky part is that users can extend this image set with additional images of custom sizes, as described on the wiki page. I expect that they will then also have to create their own beans that map to their custom image set types. Ideally, these beans could extend the new HST beans and reuse functionality to return the additional images.