[RenameDialog] Fix how percent-encoded filenames are shown in src/dest lables
Using url.toDisplayString(QUrl::PreferLocalFile) would display a file named '%20' as '%2520' ('%' is encoded to '%25'), which is confusing to the user as this differs from the name he can see in e.g. Dolphin and in the line-edit in the RenameDialog.
toDisplayString() doesn't accept the QUrl::FullyDecoded flag, the only way to show '%20' as-is is to use toLocalFile().