These job plugins work similarly to view modules, with the following
differences:
* These jobs need to link only libcalamares, not libcalamaresui. For
this reason, PluginFactory was moved from libcalamaresui to
libcalamares. (It depends only on QtCore.)
* Instead of deriving from ViewModule, derive from CppJob (which is a
subclass of Job).
* Like process and Python jobs, a job plugin is a single job, whereas a
ViewModule can generate a whole list of jobs.
The CppJob and CppJobModule classes are new. In Module::fromDescriptor,
the combination type=job, intf=qtplugin is now supported and mapped to
CppJobModule.
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