* fixed: #665 NSNull does not implement CAAction

Corresponding code added manually as corresponding code is declared in QuartzCore as extension while NSNull is being parsed from Foundation.
Declared in QartzCore as bellow:
```
/** NSNull protocol conformance. **/
@interface NSNull (CAActionAdditions) <CAAction>
@EnD
```

compiler/cocoatouch/src/main/java/org/robovm/apple/foundation/NSNull.java

@@ -44,7 +44,7 @@
 /*<annotations>*/@Library("Foundation") @NativeClass/*</annotations>*/
 /*<visibility>*/public/*</visibility>*/ class /*<name>*/NSNull/*</name>*/ 
     extends /*<extends>*/NSObject/*</extends>*/ 
-    /*<implements>*/implements NSSecureCoding/*</implements>*/ {
+    /*<implements>*/implements NSSecureCoding/*</implements>*/, CAAction /* CAAction is added as category in QuartzCore */{
 
     /*<ptr>*/public static class NSNullPtr extends Ptr<NSNull, NSNullPtr> {}/*</ptr>*/
     /*<bind>*/static { ObjCRuntime.bind(NSNull.class); }/*</bind>*/
@@ -68,4 +68,8 @@
     @Method(selector = "initWithCoder:")
     protected native @Pointer long init(NSCoder coder);
     /*</methods>*/
+
+    /* CAAction implementation */
+    @Method(selector = "runActionForKey:object:arguments:")
+    public native void runAction(String event, NSObject anObject, NSDictionary<NSString, ?> dict);
 }