layout: fix keyboard in modal dialogs

Fix the keyboard in modal dialogs (such as the run dialog) by raising
it above the ModalDialog lightbox so you can use it to type.

https://bugzilla.gnome.org/show_bug.cgi?id=657986

js/ui/layout.js

@@ -265,6 +265,7 @@ LayoutManager.prototype = {
 
     showKeyboard: function () {
         Main.messageTray.hide();
+        this.keyboardBox.raise_top();
         Tweener.addTween(this.keyboardBox,
                          { anchor_y: this.keyboardBox.height,
                            time: KEYBOARD_ANIMATION_TIME,