window-list: Filter skip-taskbar windows

Applications now track all their windows, not just the ones that are
expected to show up in the window list. So to restore the previous
behavior, we now have to filter out windows with the skip-taskbar
hint ourselves.

https://bugzilla.gnome.org/show_bug.cgi?id=724134

extensions/window-list/extension.js

@@ -315,7 +315,7 @@ const AppContextMenu = new Lang.Class({
     _getWindowList: function() {
         let workspace = global.screen.get_active_workspace();
         return this._app.get_windows().filter(function(win) {
-            return win.located_on_workspace(workspace);
+            return !win.skip_taskbar && win.located_on_workspace(workspace);
         });
     },
 
@@ -436,7 +436,7 @@ const AppButton = new Lang.Class({
     _getWindowList: function() {
         let workspace = global.screen.get_active_workspace();
         return this.app.get_windows().filter(function(win) {
-            return win.located_on_workspace(workspace);
+            return !win.skip_taskbar && win.located_on_workspace(workspace);
         });
     },