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.
Well in commit 470ae172 a newline byte (0x0a) got added at the very end of the file. This is something some editors do apparently. Your commit actually "repaired" that, so it's in line again with all the other .csproj files. Or at least according to VS standards.

So Phrohdoh's nit is actually invalid here.

But that doesn't make it less annoying. Why are VS and monodevelop (and other linux editors) not agreeing on this?