Rewrite syncplay using a state design pattern

MediaBrowser.Controller/SyncPlay/IPlaybackGroupRequest.cs

@@ -0,0 +1,24 @@
+using System.Threading;
+using MediaBrowser.Model.SyncPlay;
+using MediaBrowser.Controller.Session;
+
+namespace MediaBrowser.Controller.SyncPlay
+{
+    /// <summary>
+    /// Interface IPlaybackGroupRequest.
+    /// </summary>
+    public interface IPlaybackGroupRequest
+    {
+        /// <summary>
+        /// Gets the playback request type.
+        /// </summary>
+        /// <value>The playback request type.</value>
+        PlaybackRequestType Type();
+
+        /// <summary>
+        /// Applies the request to a group.
+        /// </summary>
+        /// <value>The operation completion status.</value>
+        bool Apply(ISyncPlayStateContext context, ISyncPlayState state, SessionInfo session, CancellationToken cancellationToken);
+    }
+}