window-list: Do not add sticky windows more than once

When a window's on-all-workspaces property changes to true, the
workspaces the window was not located on will emit the ::window-added
signal for the window; however we don't want multiple buttons for
the same window, so filter out the extra calls.

https://bugzilla.gnome.org/show_bug.cgi?id=736398

extensions/window-list/extension.js

@@ -983,6 +983,12 @@ const WindowList = new Lang.Class({
         if (this._grouped)
             return;
 
+        let children = this._windowList.get_children();
+        for (let i = 0; i < children.length; i++) {
+            if (children[i]._delegate.metaWindow == win)
+                return;
+        }
+
         let button = new WindowButton(win);
         this._windowList.layout_manager.pack(button.actor,
                                              true, true, true,