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.

Module descriptors are now module.desc, no configuration allowed inside.
Module config files are <modulename>.conf, installed in
share/calamares/modules.
settings.conf is read from /etc/calamares, then share/calamares, and if
running with --debug also in the current dir.
Module config files are read from /etc/calamares/modules, then
share/calamares/modules, and if running with --debug also in
src/modules/<modulename> relative to the current dir.

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.

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.

Also switched to C++11.