added : ability to manage a set of elements, element by element. This
screen is very close to the existing "infos_images" (which will soon
disappear).
* elements batch management : in screen element_set_global, the display
options are displayed at the top as in element_set_unit
git-svn-id: http://piwigo.org/svn/trunk@763 68402e56-0260-453c-a942-63ccdbb3a9ee
