Implement "Enemy unit sighted" notification

OpenRA currently has no such notification, and I miss 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.