revision of plugin management page

1) Took rid of order (i don't think we need that at all)
2) added the count of uses of each plugin.
3) Enabled the delete (uninstall) only if the count of instance of plugin is 0.
3) Keep the hide/show working. That will allow to hide fields for NEW surveys
4) added icons in compliance with http://myserver/HEAD/admin/qtypes.php