destDir is supposed to be a local path, so treat it as such and use
QUrl::fromLocalFile() to convert it to QUrl.
And use QUrl::path() to get the directory as QString, QUrl::toString()
returns an Url including "file://".

Also, adjust KGet::redownloadTransfer() this change, it should actually
pass a path as destDir (without "file://"), and avoid an unnecessary
QUrl->QString->QUrl conversion for the src whilst at it.

This fixes spurious "Directory is not writable" errors when trying to
add a download via the clipboard "Auto-Paste" feature or DBUS.

BUG: 390612
FIXED-IN: 17.12.3