Add support for osuTK Vector2 json serialisation

To be used by `TabletHandler`. Taken from osu! and applied globally.

osu.Framework/IO/Serialization/Vector2Converter.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 System;
+using Newtonsoft.Json;
+using Newtonsoft.Json.Linq;
+using osuTK;
+
+namespace osu.Framework.IO.Serialization
+{
+    /// <summary>
+    /// A type of <see cref="JsonConverter"/> that serializes only the X and Y coordinates of a <see cref="Vector2"/>.
+    /// </summary>
+    public class Vector2Converter : JsonConverter<Vector2>
+    {
+        public override Vector2 ReadJson(JsonReader reader, Type objectType, Vector2 existingValue, bool hasExistingValue, JsonSerializer serializer)
+        {
+            var obj = JObject.Load(reader);
+            return new Vector2((float)obj["x"], (float)obj["y"]);
+        }
+
+        public override void WriteJson(JsonWriter writer, Vector2 value, JsonSerializer serializer)
+        {
+            writer.WriteStartObject();
+
+            writer.WritePropertyName("x");
+            writer.WriteValue(value.X);
+            writer.WritePropertyName("y");
+            writer.WriteValue(value.Y);
+
+            writer.WriteEndObject();
+        }
+    }
+}

osu.Framework/Platform/GameHost.cs

···
 using System.Runtime.InteropServices;
 using System.Threading;
 using System.Threading.Tasks;
+using Newtonsoft.Json;
 using osuTK;
 using osuTK.Graphics;
 using osuTK.Graphics.ES30;
···
 using SixLabors.ImageSharp.Processing;
 using osu.Framework.Graphics.Textures;
 using osu.Framework.Graphics.Video;
+using osu.Framework.IO.Serialization;
 using osu.Framework.IO.Stores;
 using SixLabors.ImageSharp.Memory;
 using Image = SixLabors.ImageSharp.Image;
···
         protected GameHost(string gameName = @"")
         {
             Name = gameName;
+
+            JsonConvert.DefaultSettings = () => new JsonSerializerSettings()
+            {
+                Converters = new List<JsonConverter>()
+                {
+                    new Vector2Converter()
+                }
+            };
         }
 
         /// <summary>