Fix the new-widgets addon list dialog.

This was probably broken by the commit that sets the id of the window
definition to the instance, didn't test it but quite sure.

changelog

@@ -13,6 +13,7 @@ Version 1.7.7+svn:
    * Fix not being able to cancel the different version message (bug #14438)
    * Fix not being able to close corrupted file dialog (bug #13764, #14058)
    * Improve display order of unit healing (patch #1343)
+   * Fix the new-widgets addon list dialog
 
 Version 1.7.7:
  * AI:

data/gui/default/window/addon_list.cfg

@@ -90,7 +90,7 @@
 					vertical_grow = "true"
 
 					[listbox]
-						id = "addon_list"
+						id = "addons"
 						definition = "default"
 						[header]
 

src/gui/dialogs/addon_list.cpp

@@ -32,7 +32,7 @@ namespace gui2 {
  * construction and only used with --new-widgets.
  *
  * @start_table = grid
- *     (addon_list) (listbox) ()  A listbox that will contain the info about
+ *     (addons) (listbox) ()      A listbox that will contain the info about
  *                                all addons on the server.
  *     -[name] (control) ()       The name of the addon.
  *     -[version] (control) ()    The version number of the addon.
@@ -50,7 +50,7 @@ twindow* taddon_list::build_window(CVideo& video)
 
 void taddon_list::pre_show(CVideo& /*video*/, twindow& window)
 {
-	tlistbox& list = find_widget<tlistbox>(&window, "addon_list", false);
+	tlistbox& list = find_widget<tlistbox>(&window, "addons", false);
 
 	/**
 	 * @todo do we really want to keep the length limit for the various