Moved: BaseBuilding out of core => RA

Implemented: INotifyKeyPress for World traits (to respond on key pressed)
Moved: GotoNextBase (backspace key) out of core => RA
Added: GotoNextBase trait to both RA & CNC

OpenRA.Mods.RA/BaseBuilding.cs

@@ -0,0 +1,8 @@
+using OpenRA.Traits;
+
+namespace OpenRA.Mods.RA
+{
+	/* tag trait for "bases": mcv/fact */
+	public class BaseBuildingInfo : TraitInfo<BaseBuilding> { }
+	public class BaseBuilding { }
+}