refactor: introduce "view models"

This change prepares Keysmith for moving logic away from QML to C++

 - Added view model classes for each defined Navigation::Page instance
 - Added 'flows' to provide a C++ equivalent for control flow logic
   which currently still resides in QML

The purpose the view model classes is to provide data (properties) and
actions (methods to invoke) to the QML page UI. These are relatively
thin wrappers to expose the C++ state (Store) and logic (flows) as an
easy to use API for the QML UI.