Get rid of `Dispatcher.Update()` for multithreaded tile fetching

[wilhelmberg]

some ideas:

• use Task<>, https://msdn.microsoft.com/en-us/library/dd537614.aspx
• use AsyncOperation or SynchronizationContext
  • https://lostechies.com/gabrielschenker/2009/01/23/synchronizing-calls-to-the-ui-in-a-multi-threaded-application/
  • http://stackoverflow.com/a/1739320
  • https://msdn.microsoft.com/en-us/library/system.componentmodel.asyncoperation.aspx
  • https://msdn.microsoft.com/en-us/library/system.threading.synchronizationcontext(v=vs.110).aspx
  • http://stackoverflow.com/a/9377740
  • http://stackoverflow.com/a/22655026
• use BackgroundWorker, https://msdn.microsoft.com/en-us/library/system.componentmodel.backgroundworker.aspx
• Implement ISynchronizeInvoke, http://stackoverflow.com/a/6708948

[wilhelmberg]

Existing multithreading has been removed: #54

Next iteration will be based on Task as System.Threading.Thread is not available with UWP.