Add iOS clipboard implementation

osu.Framework.iOS/IOSClipboard.cs

···
+// Copyright (c) ppy Pty Ltd <[e-mail]>. Licensed under the MIT Licence.
+// See the LICENCE file in the repository root for full licence text.
+
+using osu.Framework.Platform;
+using UIKit;
+
+namespace osu.Framework.iOS
+{
+    public class IOSClipboard : Clipboard
+    {
+        private readonly IOSGameView gameView;
+
+        internal IOSClipboard(IOSGameView gameView)
+        {
+            this.gameView = gameView;
+        }
+
+        public override string GetText()
+        {
+            string text = "";
+            gameView.InvokeOnMainThread(() => text = UIPasteboard.General.String);
+            return text;
+        }
+
+        public override void SetText(string selectedText) => gameView.InvokeOnMainThread(() => UIPasteboard.General.String = selectedText);
+    }
+}

osu.Framework.iOS/IOSGameHost.cs

···
 
         public override void OpenUrlExternally(string url) => throw new NotImplementedException();
 
+        public override Clipboard GetClipboard() => new IOSClipboard(gameView);
+
         public override IResourceStore<TextureUpload> CreateTextureLoaderStore(IResourceStore<byte[]> underlyingStore)
             => new IOSTextureLoaderStore(underlyingStore);