qt: Add mechanism for missing d-pointer in Job

* lang/qt/src/job_p.h: New.
* lang/qt/src/job.cpp (typedef JobPrivateHash, d_func, setJobPrivate,
getJobPrivate): New.

Because of ABI compatibility requirements we cannot add a d-pointer
to Job. Therefore we store the d-pointers in a global static. This
mechanism will allow Job subclasses to store additional data without
breaking the ABI.

• GnuPG-bug-id: T4717