Add place variant building for BaseBuilderBotModule.

1. If it follow the refinery placing logic, then we can use Facings in PlaceBuildingVariants to help BaseBuilderBotModule "rotates" it to minefield.

2. If it is a normal building, BaseBuilderBotModule will place a random variant actor.

OpenRA.Mods.Common/Traits/Buildings/PlaceBuildingVariants.cs

@@ -21,6 +21,9 @@ namespace OpenRA.Mods.Common.Traits
 		[Desc("Variant actors that can be cycled between when placing a structure.")]
 		public readonly string[] Actors = null;
 
+		[Desc("Facing of the non-variant actor, followed by facings for each variant actor. The length equals the length of Actors + 1.")]
+		public readonly WAngle[] Facings = null;
+
 		public override object Create(ActorInitializer init) { return new PlaceBuildingVariants(); }
 	}