Extract auto-selection-size into its own interface.

OpenRA.Game/Traits/TraitsInterfaces.cs

@@ -35,6 +35,7 @@ namespace OpenRA.Traits
 
 	public interface ITick { void Tick(Actor self); }
 	public interface IRender { IEnumerable<Renderable> Render(Actor self); }
+	public interface IAutoSelectionSize { int2 SelectionSize(Actor self); }
 
 	public interface IIssueOrder
 	{