Fix for empty filename on using CodeImpThread::emitMessageToLog from importers:

umbrello/codeimpwizard/codeimpthread.h
- In explicit constructor pass argument `file' by reference.
- Change private member m_file to reference.

umbrello/codeimpwizard/codeimpthread.cpp
- In function emitMessageToLog, if the `file' argument is passed in
  empty then provide m_file.fileName() to signal messageToLog().