Refactoring multithreading

1. ThreadPool -> Task
2. fix deadlock
3. remove unnecessory application.dispatcher.invoke
4. enable non-main thread access to results collection
5. Misc
6. part of #412

Wox/SettingWindow.xaml.cs

@@ -47,7 +47,7 @@ namespace Wox
             InitializeComponent();
             _settings = settings;
             _api = api;
-            ResultListBoxPreview.DataContext = new ResultsViewModel(_settings, null);
+            ResultListBoxPreview.DataContext = new ResultsViewModel(_settings);
             Loaded += Setting_Loaded;
         }