GWT: x.x, Edition, add zoom and drag utilities to facsimile images

See the spec (FR): https://groupes.renater.fr/wiki/txm-info/public/specs_production_editions
By default, the image must fit the tab width (within min & max limits)
(cf. #596)

(from redmine: issue id 625, created on 2014/02/19 by Alexey Lavrentev)