page['category'] is not an id anymore, but an associative array of category info
all of page['cat_xxx'] or page['uppercats'] merged into one
simplifies calls to make_index_url
give plugins a clean start for page variables for version 1.7
git-svn-id: http://piwigo.org/svn/trunk@1861 68402e56-0260-453c-a942-63ccdbb3a9ee