Qt: Add DefaultKeyGenerationJob

* lang/qt/src/defaultkeygenerationjob.cpp,
lang/qt/src/defaultkeygenerationjob.h: New.
* lang/qt/src/Makefile.am: Update accordingly.

The defaultkeygenerationjob makes it easier to just generate a key
in the future this should probably use quick-gen key but since this
is not exposed in gpgme we hardcode the defaults and do it with
the params file.

This is also the first job that uses a new and better architecture
without backend abstraction and the pimpl pattern instead of
a specialized subclass.

This is an adoption of kde's libkleo commit f49b7157
Thanks dvratil@kde.org