Refactor JSBridge to simplify adding new methods. Add log(message) and a stub for fileExistsInMod(file,mod).

OpenRA.Launcher.Mac/JSBridge.h

@@ -12,7 +12,11 @@
 @class Controller;
 @interface JSBridge : NSObject {
 	Controller *controller;
+	NSDictionary *methods;
 }
+@property(readonly) NSDictionary *methods;
+
++ (JSBridge *)sharedInstance;
+- (void)setController:(Controller *)aController;
 
--(id)initWithController:(Controller *)aController;
 @end