Check for m_tempDirSelectionWidget->tempPath() and m_tempDirSelectionWidget-tempPath() + ".toc"

For example, if /tmp/k3b.iso and /tmp/k3b.iso.toc exist, then it
needs to ask user 'Do you want to overwrite XXX?' should
K3b::MediaCopyDialog automatically delete tmp files? I argue that
it is better to mv tmp files by user.

CCBUG: 379849