Move UI code into a separate platform-specific folder

Later, we hope to have a UI/macOS folder for developing a macOS client
from this codebase.

Signed-off-by: Roopesh Chander <roop@roopc.net>

WireGuard/WireGuard/UI/iOS/AppDelegate.swift

@@ -0,0 +1,28 @@
+//
+//  AppDelegate.swift
+//  WireGuard
+//
+//  Created by Roopesh Chander on 11/08/18.
+//  Copyright © 2018 Roopesh Chander. All rights reserved.
+//
+
+import UIKit
+
+@UIApplicationMain
+class AppDelegate: UIResponder, UIApplicationDelegate {
+
+    var window: UIWindow?
+
+    func application(_ application: UIApplication,
+                     didFinishLaunchingWithOptions launchOptions: [UIApplicationLaunchOptionsKey: Any]?) -> Bool {
+
+        let window = UIWindow(frame: UIScreen.main.bounds)
+        window.backgroundColor = UIColor.white
+        self.window = window
+
+        window.rootViewController = MainViewController()
+        window.makeKeyAndVisible()
+
+        return true
+    }
+}