window-list: Don't consider skip-taskbar windows for app sorting

It is odd to consider windows that are not shown in the window list
for app sorting, in particular when switching between grouped and
ungrouped mode, and when a long-lived window like the DESKTOP is
present.

https://bugzilla.gnome.org/show_bug.cgi?id=753055

extensions/window-list/extension.js

@@ -57,7 +57,8 @@ function _onMenuStateChanged(menu, isOpen) {
 }
 
 function _getAppStableSequence(app) {
-    return app.get_windows().reduce(function(prev, cur) {
+    let windows = app.get_windows().filter(function(w) { return !w.skip_taskbar; });
+    return windows.reduce(function(prev, cur) {
         return Math.min(prev, cur.get_stable_sequence());
     }, Infinity);
 }