NSURL: create .fileURL property

Create the NSURL.fileURL property to increase compatibility with
applications that rely on this.

Headers/Foundation/NSURL.h

@@ -171,6 +171,12 @@ enum
 - (id) initWithString: (NSString*)aUrlString
 	relativeToURL: (NSURL*)aBaseUrl;
 
+#if GS_HAS_DECLARED_PROPERTIES
+@property (readonly, getter=isFileURL) BOOL fileURL;
+#else
+- (BOOL) isFileURL;
+#endif
+
 /**
  * Returns the full string describing the receiver resolved against its base.
  */
@@ -214,11 +220,6 @@ enum
  */
 - (NSString*) host;
 
-/**
- * Returns YES if the receiver is a file URL, NO otherwise.
- */
-- (BOOL) isFileURL;
-
 #if OS_API_VERSION(MAC_OS_X_VERSION_10_6,GS_API_LATEST) 
 /** Returns the last (rightmost) path component of the receiver.
  */