cpp: Add support for setting owner trust and for disabling keys
* lang/cpp/src/context.cpp, lang/cpp/src/context.h (class Context): Add member functions setOwnerTrust, startSetOwnerTrust, setKeyEnabled, startSetKeyEnabled. * lang/cpp/src/context.cpp (owner_trust_to_string): New.
- GnuPG-bug-id: T7239