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/modalDialog.js

@@ -56,7 +56,7 @@ const ModalDialog = new Lang.Class({
                                       x: 0,
                                       y: 0,
                                       accessible_role: Atk.Role.DIALOG });
-        Main.uiGroup.add_actor(this._group);
+        Main.layoutManager.modalDialogGroup.add_actor(this._group);
 
         let constraint = new Clutter.BindConstraint({ source: global.stage,
                                                       coordinate: Clutter.BindCoordinate.ALL });