MimeTypeFinderJob: the StatJob details should include the mimetype

Apparently we forgot to specify that we want the UDS_MIME_TYPE field in
the statFile() method (both when it lived in OpenUrlJob and when it was moved
to MimeTypeFinderJob). And now there is a dedicated StatJob flag, StatMimeType,
that we can use.

Not passing KIO::StatMimeType when creating the StatJob meant the code always
used a get job to determine the mime type, which mean that e.g. opening an
ISO file from Dolphin, which supposedly just needs to launch Ark, had the
whole file read into memory, which means that opening a couple of ISO's and
you're out of memory...

Thanks to sitter for doing a big chunk of the investigative work in the bug
report.

CCBUG: 398908