Add rotatable property to artillery projectile

[calcwizard]

Normal projectiles have a rotatable property that controls whether the projectile is rotated to match its direction of travel. Artillery projectiles don't have this property but use the behavior by default. While this works well for the vanilla projectile, it looks out of place when using something like a disc, which looks better when pointed north. I would appreciate it if this behavior could be disabled for artillery projectiles as well.

Thanks in advance.

[Bilka]

Okay, added for the next version.