Gallery Manager improvements

Gallery manager currently has the following flaws:

• UI exposes variant name translations, but these do not work. There may be a related problem in the archetype / product: the translations equally don't work for standard hippogallery:imageset. (tje: I investigated and created CMS7-8086 for this.)
• When images are converted into a new imageset, no-longer existing variants are not removed, they linger.
• The updating of image sets is global. The UI should expose more granularity in order to limit the set of to be converted image sets.
• When checking the result of the image set conversion in the CMS without re-login, the CMS shows an error message at the bottom of the page. If this can't be avoided, the UI should point the user to re-login.
• When you select an imageset and change the width/heigth for multiple variants, then press save, only the width/height for one of the variants is saved. Expected: the width/height of all variants in the imageset are saved. This is relevant for the Hippo Trail sprint 1 (http://www.onehippo.org/trails/developer-trail/configure-out-of-the-box-features/configure-image-variants.html).