Do not sort the directory entry list in listNew() and listCurrent():

According to callgrind dumps at

https://bugs.kde.org/show_bug.cgi?id=334218#c4

QDir sorts the list of directory entries unless specified otherwise.

After disabling the sorting the callgrind is quite different already:

https://bugs.kde.org/show_bug.cgi?id=334218#c7

This already helps shortening the time to synchronize folders visibly,
but KMail gets still blocked for half a minute or more. There is another
occurence of QDir entry sorting in keycache.cpp which the next commit
will address.

Thanks to Sergio and David for help and pointing out how to disable
the sorting.

CCBUG: 334218

REVIEW: 117975

DIGEST: Huge performance improvement for POP3 users with large maildir.