Fix 'Show Target' for non-local URLs

Links are not always local files, e.g. 'remote:/zeroconf' links to 'zeroconf:/'

The current code fails there

Instead of using QFile API that does not work on non-local URLs use QUrl::resolved to resolve relative links

Furthermore, QFile::exists doesn't work for non-local URLs. Instead do a KIO stat to check whether the target exists