Create a new type of bullet: "Grenade"

[JerryLin828]

Grenade

The grenade (手榴弹) is a classic weapon in war games.

Unlike the original bullet (cannonball), it should have the following traits:

• Its traveling speed decreases over time (to imitate the reaction it has when being thrown and bouncing on the ground).
• Greater damage (3.0x) than the cannonball, with a larger damage radius (all tanks within the range, including the tank that threw the grenade, will be damaged).
• A delayed explosion (3 seconds countdown before it explodes).