window-list: update for gnome-shell changes

Shell.WindowTracker.is_window_interesting() was removed, and
replaced with the skip_taskbar MetaWindow property.

extensions/window-list/extension.js

@@ -876,7 +876,7 @@ const WindowList = new Lang.Class({
     },
 
     _onWindowAdded: function(ws, win) {
-        if (!Shell.WindowTracker.get_default().is_window_interesting(win))
+        if (win.skip_taskbar)
             return;
 
         if (this._grouped)