Isolate platform SDK includes from app header

src/app.h

@@ -32,23 +32,23 @@ struct PlatformStoragePaths;
 }
 
 #if defined(__OBJC__) && defined(__IOS__)
-#import <Foundation/Foundation.h>
-#import "GameViewController.h"
-#import "AppDelegate.h"
+@class GameViewController;
+@class AppDelegate;
 #endif
 
 #if defined(__OBJC__) && defined(__OSX__)
-#import "main.h"
+@class View;
+@class AppOSX;
 #endif
 
 #ifdef __ANDROID__
-#include "main.h"
+struct android_app;
+struct engine;
 #endif
 
-#ifdef __LINUX__
-#include <GLFW/glfw3.h>
+#if __LINUX__ || __WEB__
+struct GLFWwindow;
 #endif
-
 struct VRController
 {
     enum class kButton : uint8_t