cpp: Add gpgme_data_rewind to cpp API

* lang/cpp/src/data.h, lang/cpp/src/data.cpp (Data::rewind): New.
* lang/qt/tests/t-various.cpp (testDataRewind): Test it.

The advantage of this convieniance function in GPGME is that
it avoids the messiness that are declarations with off_t.

• GnuPG-Bug-Id: T3996