app: Set styleclass on fallback app icons

For window backed apps, create_icon_texture() doesn't return an StIcon
but a generic widget. Set an appropriate style class to make it easier
to apply a specific style only to fallback icons.

https://gitlab.gnome.org/GNOME/gnome-shell/issues/1027

src/shell-app.c

@@ -215,6 +215,8 @@ window_backed_app_get_icon (ShellApp *app,
                                                          G_OBJECT (window),
                                                          "icon",
                                                          size);
+  st_widget_add_style_class_name (widget, "fallback-app-icon");
+
   return CLUTTER_ACTOR (widget);
 }