Feeds: pull request regarding RSS image dimensions

https://github.com/bloomreach-forge/feeds/pull/2

Image width and height are required and don't validate because set by default to -1 and not set to the right value with the hst/src/main/java/org/bloomreach/forge/feed/api/transform/rss/HippoGalleryImageSetToImageTransformer.java

Another thing is, that the image title should be editable, so I changed the title from file name to image description which can be edited in the CMS gallery.

Note, this regards the images on RSS document level, so the <rss><channel><image> element in the feed.