Remove RPC from main distribution (#925)

experiment/Discord.Net.Rpc/Entities/Messages/RpcSystemMessage.cs

@@ -0,0 +1,33 @@
+using Discord.Rest;
+using System.Diagnostics;
+using Model = Discord.API.Rpc.Message;
+
+namespace Discord.Rpc
+{
+    [DebuggerDisplay(@"{DebuggerDisplay,nq}")]
+    public class RpcSystemMessage : RpcMessage, ISystemMessage
+    {
+        public MessageType Type { get; private set; }
+
+        internal RpcSystemMessage(DiscordRpcClient discord, ulong id, RestVirtualMessageChannel channel, RpcUser author)
+            : base(discord, id, channel, author, MessageSource.System)
+        {
+        }
+        internal new static RpcSystemMessage Create(DiscordRpcClient discord, ulong channelId, Model model)
+        {
+            var entity = new RpcSystemMessage(discord, model.Id,
+                RestVirtualMessageChannel.Create(discord, channelId),
+                RpcUser.Create(discord, model.Author.Value, model.WebhookId.ToNullable()));
+            entity.Update(model);
+            return entity;
+        }
+        internal override void Update(Model model)
+        {
+            base.Update(model);
+
+            Type = model.Type;
+        }
+
+        private string DebuggerDisplay => $"{Author}: {Content} ({Id}, {Type})";
+    }
+}