Score Multiplier, Health/Game Over Screen, ...

Every cannon ball that hits an enemy increase multiplier by 1. A cannon
ball that doesn't hit anything resets the multiplier to the base
(default 1).

Infantry now deal 1 damage to the Player when they reach the hill. The
Player starts off with 2 health points (displayed in the top left
corner).
When now health points remain a Game Over screen appears, press any key
to retry.

Fixed DisplayObject's Clear function not working correctly.
Changed Player Controls to use Event Handlers.
Projectiles and Explosions are now added to Layer_Other (since
Layer_Objects is used for collision tests).