Allow PICKUP flag to be used with projectiles

[GFD]

I'd love to be able to use the PICKUP actor flag to let projectiles grab items on behalf of their target. This could be used for boomerang-like items, or to emulate the ability of the Superball from Super Mario Land, for example.

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Less serious about this feature request, but it'd also be neat if you could define Pickup.* states (for actors that aren't inheriting from the native CustomInventory class) that actors enter when they pick up an item; could be useful for giving actors animations when they pick up something, or special stuff like making a ZombieMan turn into a ShotgunGuy if it picks up a shotgun (using a Pickup.Shotgun state).