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

Plugins/Wox.Plugin.Program/FileChangeWatcher.cs

@@ -1,6 +1,7 @@
 using System.Collections.Generic;
 using System.IO;
 using System.Threading;
+using System.Threading.Tasks;
 using Wox.Infrastructure.Logger;
 
 namespace Wox.Plugin.Program
@@ -40,7 +41,7 @@ namespace Wox.Plugin.Program
         {
             if (!isIndexing)
             {
-                ThreadPool.QueueUserWorkItem(o =>
+                Task.Run(() =>
                 {
                     Programs.IndexPrograms();
                     isIndexing = false;

Plugins/Wox.Plugin.Program/ProgramSetting.xaml.cs

@@ -1,5 +1,5 @@
 using System.IO;
-using System.Threading;
+using System.Threading.Tasks;
 using System.Windows;
 using System.Windows.Controls;
 
@@ -31,7 +31,7 @@ namespace Wox.Plugin.Program
         private void ReIndexing()
         {
             programSourceView.Items.Refresh();
-            ThreadPool.QueueUserWorkItem(t =>
+            Task.Run(() =>
             {
                 Dispatcher.Invoke(() => { indexingPanel.Visibility = Visibility.Visible; });
                 Programs.IndexPrograms();