Rewrite syncplay using a state design pattern

MediaBrowser.Model/SyncPlay/GroupState.cs

@@ -0,0 +1,25 @@
+namespace MediaBrowser.Model.SyncPlay
+{
+    /// <summary>
+    /// Enum GroupState.
+    /// </summary>
+    public enum GroupState
+    {
+        /// <summary>
+        /// The group is in idle state. No media is playing.
+        /// </summary>
+        Idle,
+        /// <summary>
+        /// The group is in wating state. Playback is paused. Will start playing when users are ready.
+        /// </summary>
+        Waiting,
+        /// <summary>
+        /// The group is in paused state. Playback is paused. Will resume on play command.
+        /// </summary>
+        Paused,
+        /// <summary>
+        /// The group is in playing state. Playback is advancing.
+        /// </summary>
+        Playing
+    }
+}