GUI: Clear results from view before starting new compare.

Strictly speaking this is not necessary but it is the easiest way to set the GUI state to "no results"-state.

gui/mainwindow.cpp

@@ -241,6 +241,8 @@ void MainWindow::DoCheckFiles(QFileDialog::FileMode mode)
 
     if (selected.count() > 0)
     {
+        ClearResults();
+
         QStringList fileNames;
         QString selection;