Improve status reporting

There are some user complaints that Imager hangs at "writing 0%"
Mention in the progress status what it is actually doing prior
to writing (e.g. opening storage, blkdiscarding,
zero'ing out MBR, starting download) to help diagnose
potential issues.

localfileextractthread.cpp

@@ -31,6 +31,7 @@ void LocalFileExtractThread::run()
     if (isImage() && !_openAndPrepareDevice())
         return;
 
+    emit preparationStatusUpdate(tr("opening image file"));
     _timer.start();
     _inputfile.setFileName( QUrl(_url).toLocalFile() );
     if (!_inputfile.open(_inputfile.ReadOnly))