Implement levels

[emurnane]

The game currently provides options for playing Level 1 or Level 2, which are both 60 seconds.

• Instead, replace that screen with the instructions page (see issue Add an instructions page #2).
• Then, automatically start the player at Level 1, which should be displayed at the top center of the screen so the player knows what level they're currently on. Level 1 should allot 60 seconds.
• If they clear Level 1, progress to Level 2, which allows 50 seconds.
• As the player clears the board, continue progressing through the levels (which should be updated at the top of the screen) by decrementing the time by 10 seconds. (Level 3 is 40 seconds, Level 4 is 30 seconds, and so on).