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.

In order to avoid deep copies, Qt containers over which we iterate must be const
The remaining unported cases require qAsConst (Qt 5.7) or std::as_const (C++17)

This also adds ImageRegistry, a SVG cache, a bunch of utility functions
for shared pixmaps in CalamaresUtilsGui and renames several qrc files
in viewmodules.