qt: Add API to get the context for a Job

* lang/qt/src/job.cpp,
lang/qt/src/job.h (Job::context): New.
* lang/qt/src/threadedjobmixin.cpp
(ThreadedJobMixin::~ThreadedJobMixin): New. Unregister context.
(ThreadedJobMixin::lateInitialization): Register context.
* NEWS: Update for cpp and qt.

The global map hack is necessary because the job class does
not know about the context which is held in threadedjobmixin.
Adding a virtual function in Job would be an ABI break which
I'd like to avoid at this point. Although QGpgME's API will
need a big ABI break to make it ABI maintainable. The virtual
jobs -> implementation classes are nearly unmaintainable ABI wise.

The context is exposed to provide more flexibility to users, e.g.
to add a passphrase callback or to set the sender in a context.