Implement "Enemy unit sighted" notification

OpenRA currently has no such notification, and I think it needs it.
This adds a trait for the Player actor to track all neutral/enemy units that the player sees and a trait for units/structures/creeps to announce themselves when seen.
Currently added to D2k units and the sandworms.
Not added to D2k structures and TD/RA/TS mods because I don't have the appropriate notifications. If someone could assist me with those I'll add them.
Also removed the "WormSign" notification on worm spawn, since it never belonged there.
Use the Actor overload of Shroud.IsVisible instead.