Auto-Organize: Async operation and instant feedback UI (reworked)

This commit includes changes to enable and stabilize asyncronous
operation in the auto-organize area. Here are the key points:

- The auto-organize correction dialog is now closed (almost) instantly.
This means that the user does not have to wait until the file copy/move
operation is completed in order to continue. (even with local HDs the
copy/move process can take several minutes or even much longer with
network destination).
- This commit also implements locking of files to be organized in order
to prevent parallel processing of the same item. In effect, there can be
2 or more manual organization operations active even while the normal
auto-organization task is running without causing any problems
- The items that are currently being processed are indicated as such in
the log with an orange color and a spinner graphic
- The client display is refreshed through websocket messages
- A side effect of this is that other clients showing the auto-organize
log at the same time are always up-to-date as well

MediaBrowser.Server.Implementations/Session/SessionManager.cs

@@ -1869,6 +1869,27 @@ namespace MediaBrowser.Server.Implementations.Session
             return GetSessionByAuthenticationToken(info, deviceId, remoteEndpoint, null);
         }
 
+        public Task SendMessageToAdminSessions<T>(string name, T data, CancellationToken cancellationToken)
+        {
+            // TODO: How to identify admin sessions?
+            var sessions = Sessions.Where(i => i.IsActive && i.SessionController != null).ToList();
+
+            var tasks = sessions.Select(session => Task.Run(async () =>
+            {
+                try
+                {
+                    await session.SessionController.SendMessage(name, data, cancellationToken).ConfigureAwait(false);
+                }
+                catch (Exception ex)
+                {
+                    _logger.ErrorException("Error sending message", ex);
+                }
+
+            }, cancellationToken));
+
+            return Task.WhenAll(tasks);
+        }
+
         public Task SendMessageToUserSessions<T>(string userId, string name, T data,
             CancellationToken cancellationToken)
         {