qt,cpp: Consistently use nullptr and override
* lang/cpp/src/Makefile.am, lang/qt/src/Makefile.am (AM_CPPFLAGS): Add suggest-override and zero-as-null-pointer-constant warnings. * lang/cpp/src/*, lang/qt/src/*: Consistenly use nullptr and override.
This was especially important for the headers so that downstream
users of GpgME++ or QGpgME do not get flooded by warnings if
they have these warnings enabled.
It also improves compiler errors/warnings in case of accidental