Add a reset progress button

[atorch]

The app used to have a reset progress button, but it is no longer accessible. Bring it back. This has been requested in several reviews.

The button should return the app to the initial state where none of the puzzles have been solved (i.e. delete/reset the user's progress).

[atorch]

This issue is a duplicate of #14. Apparently resetting the app is already possible (with something like Settings -> Apps -> ... Clear app data?), but an in-app button would be more user friendly.