Convert QByteArray to QString using QString::fromLatin1().

Under Qt5 direct translation from QByteArray to QString using the constructor
QString::QString (const QByteArray & ba) eats some bytes, ending up with a string
smaller than the original QByteArray, this affects the IMAP and POP protocol
implementation, since it will read more bytes than the actually message size.

This fix was tested under Qt4 for backward compatibility.

Change-Id: Ie8d7960ec169c4d9ad75898fbf06f7ea5aa424df
Reviewed-by: Matthew Vogt <matthew.vogt@qinetic.com.au>
Reviewed-by: Robin Burchell <robin+qt@viroteck.net>