Qt: Undeprecate QByteArray based start functs

* lang/qt/src/encryptjob.h, lang/qt/src/signjob.h (start):
Undeprecate QByteArray based functions.

While the QIODevice access might avoid a copy it is more
difficult to manage the data and lifetime of the
QIODevices in calling code. The QByteArray calls are
convienient for small data objects like mails where
an in memory copy is not really expensive anymore.