Revert "add Screen class singleton"

This reverts commit 6075a915028e21bffb61b1015414a61798d012d7.

Conflicts:
	src/it/unical/mat/igpe/graphics/CenterGamePanel.java
	src/it/unical/mat/igpe/graphics/KeyProcessor.java
	src/it/unical/mat/igpe/ultimateDisc/movingObject/Disc.java