qt: Allow checking if the sign/encrypt archive jobs are supported
* lang/qt/src/encryptarchivejob.cpp, lang/qt/src/encryptarchivejob.h (isSupported): New. * lang/qt/src/signarchivejob.cpp, lang/qt/src/signarchivejob.h (isSupported): New. * lang/qt/src/signencryptarchivejob.cpp, lang/qt/src/signencryptarchivejob.h (isSupported): New. * lang/qt/tests/run-encryptarchivejob.cpp (main), lang/qt/tests/run-signarchivejob.cpp (main): Check if jobs are supported.
- GnuPG-bug-id: T6342