A new screen is needed to display the current list of challenges (in and out).

It should be similar to the already existing correspondence challenges list (see:

• User created (outgoing) challenges can be canceled.
• Incoming challenges can be declined or accepted

This screen should be accessed from a widget (which would look like a ListTile) on the "Play" tab main screen, below the "X games in play". This widget only show the first challenge, and has a "More" button to display the full screen if needed.

Screen and widget data is handled by a challengesProvider which is roughly described in #438 .

TODO: describe the widget.