0.2.0_alpha: high scores and new achievements

With this new minor update for TailTrail, I tried to introduce an incentive for user to keep playing the game.

Now high-scores for each difficulty level get saved for you to try to improve on them.

Also, new objectives have been added to make gameplay a bit more challenging. In a future update, passing these objectives will unlock some customization options.

Finally, a score counter has been added in-game, that can be turned off at will.

Thanks for passing by!