Improve UrlLoader behavior
This fixes two issues:
- a bug causing URLs with schemes like ftp to always be opened in external applications. This happens because for these protocols KIO::OpenUrlJob doesn't emit the mimeTypeFound signal. To fix this, we explicitly use KIO::MimeTypeFinderJob to determine the mimetype of the URL
- a bug causing an attempt to open local URLs corresponding to nonexisting paths using the smb ioslave. This is fixed by explicitly checking whether the file exists and assigning it an "inode/directory" if it doesn't exist. This way, the URL is opened by whatever part/application the user chose to display directories (of course, that part or application will then fail to open the path).