Fixed crash on switching the application language.

This crash was caused by the RemovableDriveManager and 3DConnexion
detection services not being stopped correctly.
The fix executes all the tasks of the MainFrame at the language switch,
that are normally performed on application shut down.

src/slic3r/GUI/RemovableDriveManager.cpp

@@ -434,6 +434,7 @@ void RemovableDriveManager::shutdown()
 #endif
 
 	m_initialized = false;
+	m_callback_evt_handler = nullptr;
 }
 
 bool RemovableDriveManager::set_and_verify_last_save_path(const std::string &path)