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*.
