qt: Add setInputEncoding to QGpgMe::EncryptJob

* lang/qt/src/encryptjob.cpp, lang/qt/src/encryptjob.h,
lang/qt/src/encryptjob_p.h: Add inputEncoding/setInputEncoding
to EncryptJob
* lang/qt/src/qgpgmeencryptjob.cpp: Use newly added inputEncoding
to set encoding hint of the encrypted content

This allows applications like KMail to set the input encoding of the
encrypted content, which simplify and improve the performance of
identifying the content type then decrypting it.

• GnuPG-bug-id: T6616
• Signed-off-by: Carl Schwan <carl.schwan@gnupg.com>