Fix showing of dialog the user can't interact with when saving

The issue is using KIO::move + exec() can show a dialog but exec says
to QEventLoop::ExcludeUserInputEvents so the user is stuck there
forever.

Move to KIO::file_move since we're moving files anyway and set overwrite
since we're already asking if the file exists anyway (twice in fact, fix
coming in the next commit)

BUGS: 437574