navigation to/from the Add screen

[valsen]

The page for adding a new todo is currently just a tab of the app's only page. Navigating to it by tapping its icon in the bottom tab bar works, but there is currently no way to automatically navigate back to the previously active tab upon saving or canceling the new todo.

Suggested solution:
Implement the add screen as a new page with its own route instead. This would allow popping the route from the navigation stack when saving or canceling the new todo, bringing the user back to the previous view and as a bonus reset the state of the add screen (clear all user inputs).