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)

Actual module ViewSteps can reimplement it to read their own module-
specific configuration.

Also added onActivate/onLeave to ViewStep and ViewManager.

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*.

Added ViewManager.
Added dummy Settings class.
Added dummy plugin interface (UI plugins only).
Added dummy greeting plugin.
Added DLLEXPORT macros for UI plugin interface and plugins.