Reset synchronization context on Application.Shutdown()

- Fixes https://github.com/migueldeicaza/gui.cs/issues/1084

Reset synchronization context to allow the user to run async/await, as the main loop has been ended, the synchronization context from gui.cs does no longer process any callbacks.

Terminal.Gui/Core/Application.cs

@@ -540,6 +540,12 @@ namespace Terminal.Gui {
 			Driver?.End ();
 			Driver = null;
 			_initialized = false;
+
+			// Reset synchronization context to allow the user to run async/await,
+			// as the main loop has been ended, the synchronization context from 
+			// gui.cs does no longer process any callbacks. See #1084 for more details:
+			// (https://github.com/migueldeicaza/gui.cs/issues/1084).
+			SynchronizationContext.SetSynchronizationContext (syncContext: null);
 		}
 
 		static void Redraw (View view)