qt: Add helper for converting QStringList to vector of strings

* lang/qt/src/util.h, lang/qt/src/util.cpp: New.
* lang/qt/src/Makefile.am (qgpgme_sources, private_qgpgme_headers):
Add new files.

Additionally to the internal helper function toStrings(), this adds
a copy of the internal make_error helper of GpgME++ which is used
in a few places by QGpgME. We use the same error source id as GpgME++.

• GnuPG-bug-id: T5808