add plugins window to screenshots

screenshots.php

@@ -17,6 +17,7 @@ $array_images = array(
 	"groupchat_window_thumb.png" => "groupchat_window.png",
 	"history_window_thumb.png" => "history_window.png",
 	"gajim_jingle_thumb.png" => "gajim_jingle.png",
+	"gajim_plugins_thumb.png" => "gajim_plugins.png",
 	);
 
 $tpl->set_block("page", "IMAGE", "image");