Implement a rudimentary applets system (#804)

* Implement Player Select applet

* Initialize the Horizon system reference

* Tidy up namespaces

* Resolve nits

* Resolve nits

* Rename stack to queue

* Implement an applet FIFO

* Remove debugging log

* Log applet creation events

* Reorganise AppletFifo

* More reorganisation

* Final changes

Ryujinx.HLE/HOS/Applets/PlayerSelect/PlayerSelectApplet.cs

@@ -0,0 +1,55 @@
+using Ryujinx.HLE.HOS.Services.Account.Acc;
+using Ryujinx.HLE.HOS.Services.Am.AppletAE;
+using System;
+using System.IO;
+
+namespace Ryujinx.HLE.HOS.Applets
+{
+    internal class PlayerSelectApplet : IApplet
+    {
+        private Horizon _system;
+
+        private AppletFifo<byte[]> _inputData;
+        private AppletFifo<byte[]> _outputData;
+
+        public event EventHandler AppletStateChanged;
+
+        public PlayerSelectApplet(Horizon system)
+        {
+            _system = system;
+        }
+
+        public ResultCode Start(AppletFifo<byte[]> inData, AppletFifo<byte[]> outData)
+        {
+            _inputData  = inData;
+            _outputData = outData;
+
+            // TODO(jduncanator): Parse PlayerSelectConfig from input data
+            _outputData.Push(BuildResponse());
+
+            AppletStateChanged?.Invoke(this, null);
+
+            return ResultCode.Success;
+        }
+
+        public ResultCode GetResult()
+        {
+            return ResultCode.Success;
+        }
+
+        private byte[] BuildResponse()
+        {
+            UserProfile currentUser = _system.State.Account.LastOpenedUser;
+
+            using (MemoryStream stream = new MemoryStream())
+            using (BinaryWriter writer = new BinaryWriter(stream))
+            {
+                writer.Write((ulong)PlayerSelectResult.Success);
+
+                currentUser.UserId.Write(writer);
+
+                return stream.ToArray();
+            }
+        }
+    }
+}