Background rework

New features:

• Balls are spawned at the top of the screen whenever the clicker button is clicked, these balls despawn when they reach the bottom of the screen.
• Balls are spawned at the top of the screen whenever an upgrade is purchased, these balls respawn at the top of the screen when they reach the bottom of the screen.

Fixes:

• Fixed issue where you could no longer do anything after doing a prestige or starting a new game.