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)

Now linking with -Wl,--no-undefined!
Moved find_package KF5 from the root CMakeLists.txt to the partitioning
module where it belongs. KF5 deps should be limited to plugins.
Lots of random fixage to make stuff link.
Removed CalamaresApplication reference from Settings.
Removed the use of AbstractPage, we don't need it right now.

Renamed ViewPlugin to ViewStep.
Back/Next button status management.
Subpages are now under a ViewStep's jurisdiction, we only forward back/
next signals if a ViewStep says it's got more pages to show. ViewStep
exposes a QWidget* to ViewManager, this QWidget gets added to a stack.
If a ViewStep wishes to show more than one page, it must manage more
pages on its own inside its single QWidget*.

Moved module-related classes to src/calamares/modulesystem.
Fixes to startup code path.
Made ViewPlugins load into ViewManager.
Debug code: ViewManager currently only shows a plugin's prettyName.
Added ViewModule as a subclass of Module. Other kinds of plugins should
be supported in a similar way.