Removes bottom status bar.

StatusBar class calls are commented out and replaced with notifications.
SlicicingProgress notification shows progress of slicing, ProgressIndicator notification handles other progress information, like arrange objects etc.

src/slic3r/GUI/Gizmos/GLGizmoSimplify.cpp

@@ -337,7 +337,7 @@ void GLGizmoSimplify::on_set_state()
             auto notification_manager = wxGetApp().plater()->get_notification_manager();
             notification_manager->push_notification(
                 NotificationType::CustomNotification,
-                NotificationManager::NotificationLevel::RegularNotification,
+                NotificationManager::NotificationLevel::RegularNotificationLevel,
                 _u8L("ERROR: Wait until Simplification ends or Cancel process."));
             return;
         }