QUrl::toDisplayString(PreferLocalFile) doesn't imply nor support FullyDecoded

People use url.toDisplayString(QUrl::PreferLocalFile) in messages shown to the user, assuming that this will do the right thing: show a display string for remote URL, and show toLocalFile() for local files. It's almost the case, except for local paths that contain %20.

Then it shows %2520 instead of %20.

Reference https://bugs.kde.org/show_bug.cgi?id=418984

Unittest contribution: tst_qurl.cpp.diff

For the record, the reasoning in QTBUG-41729 was "You know I consider any situation in which you lost the distinction between URL and local file to be a design flaw. So, with the understanding that any place where you use PreferLocalFile you've already lost, I don't see the harm in making it worse." – This applies to this fix as well