delete, set as default.
plugins.class.php was merged back to a state it doesn't manage themes at all.
themes.class.php was created instead, from a duplication of plugins.class.php
and strongly modified then.
feature 1507: the display of available themes is now much more "graphic".
git-svn-id: http://piwigo.org/svn/trunk@5153 68402e56-0260-453c-a942-63ccdbb3a9ee
