Documented the various overlay and animation change traits

They all kinda sound the same, are often named very generic and used hard-coded sequence names that forced people to read the source code or wait for the game to crash to find out. This should improve https://github.com/OpenRA/OpenRA/wiki/Traits a lot in that regard. Includes some minor cleanup as well.
Well, no. The sensible approach for creating Mods.Common will be to rename Mods.RA and then create a new dll for the handful of RA-specific bits.  That won't help here if you move this to the Cnc dll.