Add an axis identifier for unknown axis types and get directx to use it

if it doesn't recognise an axis on construction.

coreAPI/src/java/net/java/games/input/Component.java

@@ -273,6 +273,11 @@ public interface Component {
              */
             public static final Axis POV = new Axis("pov");
 
+            /**
+             * An unknown axis.
+             */
+            public static final Axis UNKNOWN = new Axis("unknown");
+
         }
         
         public static class Button extends Identifier {

plugins/windows/src/java/net/java/games/input/IDirectInputDevice.java

@@ -472,7 +472,7 @@ final class IDirectInputDevice {
 			case IDirectInputDevice.GUID_Button:
 				return getNextButtonIdentifier();
 			default:
-				return null;
+				return Component.Identifier.Axis.UNKNOWN;
 		}
 	}