feature 1312 added: pwg.images.setInfo can replace multiple values propreties

(tags/categories). The primary condition is to provide "tag_ids" and/or
"categories" input parameters, ie pwg.images.set won't create emptiness. You
can reduce the set of tags/categories but not remove all tags/categories.

bug 1277 fixed: with a stronger algorithm for adding/replacing categories, we
now avoid to recreate an existing image_category association.

When a remote client calls pwg.images.setInfo, Piwigo returns an error 500 if:
1. the "categories" parameter is malformed (no numeric id inside)
2. one of the input categories does not exist at database level.

git-svn-id: http://piwigo.org/svn/branches/2.0@4444 68402e56-0260-453c-a942-63ccdbb3a9ee

ws.php

@@ -289,6 +289,7 @@ function ws_addDefaultMethods( $arr )
         'default' => 0,
         'maxValue' => $conf['available_permission_levels']
         ),
+      'replace_mode' => array('default' => false),
       ),
     'POST method only. Admin only
 <br/><b>categories</b> is a string list "category_id[,rank];category_id[,rank]" The rank is optional and is equivalent to "auto" if not given.'