Fixes: QtSingleApplication: remove socket instance before listening
RevBy: Thorbjorn Details: calls the newly introduced QLocalServer::removeServer() function before listening. The problem was: When the socket file was left there after the application quit (e.g. because of not closing it or when the app crashed), the first call to listen failed with "address already in use" error. Before, listen() would delete that file in that case, but that behavior changed and the "removeServer()" method introduced instead.