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.
This is a monodevelop thing. I also end up using a hex editor every time I've changed a .csproj file. Very anoying. Also manually editing a .csproj file with vim will add that newline char - thus also needing a hex editor. Maybe we should allow this change?