Removed NioTSO client and server

- NioTSO client isn't needed because we're using RayLib
- Added FreeSO's API server to handle most backend operations

server/FSO.Server.Common/ApiAbstract.cs

@@ -0,0 +1,46 @@
+namespace FSO.Server.Common
+{
+    public class ApiAbstract
+    {
+        public event APIRequestShutdownDelegate OnRequestShutdown;
+        public event APIBroadcastMessageDelegate OnBroadcastMessage;
+        public event APIRequestUserDisconnectDelegate OnRequestUserDisconnect;
+        public event APIRequestMailNotifyDelegate OnRequestMailNotify;
+
+        public delegate void APIRequestShutdownDelegate(uint time, ShutdownType type);
+        public delegate void APIBroadcastMessageDelegate(string sender, string title, string message);
+        public delegate void APIRequestUserDisconnectDelegate(uint user_id);
+        public delegate void APIRequestMailNotifyDelegate(int message_id, string subject, string body, uint target_id);
+
+        public void RequestShutdown(uint time, ShutdownType type)
+        {
+            OnRequestShutdown?.Invoke(time, type);
+        }
+
+        /// <summary>
+        /// Asks the server to disconnect a user.
+        /// </summary>
+        /// <param name="user_id"></param>
+        public void RequestUserDisconnect(uint user_id)
+        {
+            OnRequestUserDisconnect?.Invoke(user_id);
+        }
+
+        /// <summary>
+        /// Asks the server to notify the client about the new message.
+        /// </summary>
+        /// <param name="message_id"></param>
+        /// <param name="subject"></param>
+        /// <param name="body"></param>
+        /// <param name="target_id"></param>
+        public void RequestMailNotify(int message_id, string subject, string body, uint target_id)
+        {
+            OnRequestMailNotify(message_id, subject, body, target_id);
+        }
+
+        public void BroadcastMessage(string sender, string title, string message)
+        {
+            OnBroadcastMessage?.Invoke(sender, title, message);
+        }
+    }
+}