refactor: UI (page) navigation from C++ code

An app::Navigation class is introduced with an API very similar to the
Kirigami.PageRouter in QML. This new class is responsible for pusing
populated view model classes from C++ into QML ownership and triggering
appropriate navigation events. On the QML side a signal handler
forwards these calls to the Kirigami.PageRouter to perform the actual
navigation in the UI.

This construct paves the way for moving state transition and related
logic out of QML and towards C++, since this logic is currently mostly
concerned with page navigation in Keysmith. Ultimately that transition
should make the QML (page) views more easily re-usable.