refactor: introduce store for application state

The 'Store' is a convenient wrapper for injecting a dependency on the
entire application state (root) so it can be re-used by independent
flows of C++ logic. It will be used both to provide data to the view
models for the QML UI, and as a way to record results of actions
dispatched from the UI.

With this change the following (additional) state is defined:

 - OverviewState, which will be used to track state that determines how
   the AccountsOverview QML UI should render and what features are
   enabled/disabled in it. This state may be manipulated as a a
   side-effect of various interactios with arbitrary pages.
 - FlowState, which will be used to track state about which control/
   logic flows have completed and whether or not something is currently
   running. This is will be used to avoid distracting the user from an
   active UX flow with random unrelated prompts.