Added method to set all heads
When the entity's target is set, all heads are set to that target (fixes very weird bug whereby players disconnecting and reconnecting and then being targetted by the same wither, would cause shot wither skulls to show for a second then disappear, but still explode and cause damage)
Updated to return WitherSkull shot when invoking the shoot method
Use head id's provided by the WitherHead enum instead of figuring it out every time (which can also easily break)