layout: Create a group for modal dialogs

This way we can ensure that they're always stacked under the OSK.

https://bugzilla.gnome.org/show_bug.cgi?id=719451

js/ui/layout.js

@@ -212,6 +212,10 @@ const LayoutManager = new Lang.Class({
         this.addChrome(this.trayBox);
         this._setupTrayPressure();
 
+        this.modalDialogGroup = new St.Widget({ name: 'modalDialogGroup',
+                                                layout_manager: new Clutter.BinLayout() });
+        this.uiGroup.add_actor(this.modalDialogGroup);
+
         this.keyboardBox = new St.BoxLayout({ name: 'keyboardBox',
                                               reactive: true,
                                               track_hover: true });