create app notification widget

Not sure if we want to copy Android and call this "Granite.Widgets.Toast". Just committed an example to Switchboard's Online Accounts plug. It basically looks like this:

                var close_button = new Gtk.Button.from_icon_name ("close-symbolic", Gtk.IconSize.MENU);
                close_button.get_style_context ().add_class ("close-button");
                close_button.clicked.connect (() => {
                    // Response here
                    app_notification.reveal_child = false;
                });

                notification_label = new Gtk.Label ("");
                var button = new Gtk.Button.with_label (_("Button"));
                button.clicked.connect (() => {
                    // Response here
                    app_notification.reveal_child = false;
                });

                var notification_box = new Gtk.Grid ();
                notification_box.column_spacing = 12;
                notification_box.add (close_button);
                notification_box.add (notification_label);
                notification_box.add (button);

                var notification_frame = new Gtk.Frame (null);
                notification_frame.get_style_context ().add_class ("app-notification");
                notification_frame.add (notification_box);

                app_notification = new Gtk.Revealer ();
                app_notification.margin = 3;
                app_notification.halign = Gtk.Align.CENTER;
                app_notification.valign = Gtk.Align.START;
                app_notification.add (notification_frame);

Then you stick that in an overlay and show/hide when necessary. Things to consider:

* Do we want to hard code the availability of a single button? I'm pretty sure we don't want to be able to pack arbitrary widgets. Is there a case for two buttons or should we recommend that developers use an alert in that case?

* Should these time out? On Android toasts time out