filter shown messages by focus and scope

[ssube]

When a script calls focus.show with a message, it may not be for the player: it could be for the current actor, everyone in the room, or (in the case of an item script) the actor currently holding an item. The player should not receive output meant for NPCs in other rooms, especially in a network scenario.

Pass the intended scope into focus.show, replacing the optional source with an optional scope: {source, target}. Valid targets include self, room, and world.

A container target would be cool (the actor holding this item, or room containing this actor), but depends on #53 .