modalDialog: emit 'closed' signal when dialog is closed

Opening and closing a modal dialog is not instant. There
is a transition animation involved.  When the dialog is
finished opening, it currently emits the "opened" signal.

When it's finished closing, however, it doesn't emit a
"closed" signal.  This means, there's not a good way to
know when the dialog finishes closing.

This commit adds the "closed" signal.

https://bugzilla.gnome.org/show_bug.cgi?id=694296

js/ui/modalDialog.js

@@ -265,6 +265,7 @@ const ModalDialog = new Lang.Class({
                                function() {
                                    this.state = State.CLOSED;
                                    this._group.hide();
+                                   this.emit('closed');
                                })
                          });
     },