print-editor: Set the devel style class

Lets clearly communicate that we're unstable.

demos/print-editor/print-editor.c

@@ -781,6 +781,8 @@ activate (GApplication *app)
   gtk_application_window_set_show_menubar (GTK_APPLICATION_WINDOW (main_window), TRUE);
   update_title (GTK_WINDOW (main_window));
 
+  gtk_widget_add_css_class (main_window, "devel");
+
   box = gtk_box_new (GTK_ORIENTATION_VERTICAL, 0);
   gtk_window_set_child (GTK_WINDOW (main_window), box);