Add DTR entry with number of visible pairs (#52)

* Add DTR entry with number of visible pairs

* Simplify DTR visible pair counter

- Merge the new PairHandlerInvisibleMessage and PairHandlerVisibleDisposedMessage ;
- Leave alone a bunch of messages that were actually unneccessary.

* Match encodings with the rest of the codebase

* Make the DTR entry opt-in and add explanations

* More reliable publishing of PairHandlerVisible/Invisible

* Simplify things for DTR

* Rework async

MareSynchronos/MareConfiguration/Configurations/MareConfig.cs

@@ -10,6 +10,7 @@ public class MareConfig : IMareConfiguration
     public string CacheFolder { get; set; } = string.Empty;
     public bool DisableOptionalPluginWarnings { get; set; } = false;
     public bool EnableRightClickMenus { get; set; } = true;
+    public bool EnableDtrEntry { get; set; } = false;
     public NotificationLocation ErrorNotification { get; set; } = NotificationLocation.Both;
     public string ExportFolder { get; set; } = string.Empty;
     public bool FileScanPaused { get; set; } = false;