Save when exit

1. fix image cache
2. fix save when exit since destructor won't be called
3. fix #583 #582 (partially)  #581 (partially)  #580

Wox/App.xaml

@@ -1,6 +1,8 @@
 <Application x:Class="Wox.App"
              xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
-             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
+             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+             Exit="OnExit"
+             SessionEnding="OnSessionEnding">
     <Application.Resources>
         <ResourceDictionary>
             <ResourceDictionary.MergedDictionaries>

Wox/App.xaml.cs

@@ -13,11 +13,12 @@ using Stopwatch = Wox.Infrastructure.Stopwatch;
 
 namespace Wox
 {
-    public partial class App : Application, ISingleInstanceApp
+    public partial class App : ISingleInstanceApp
     {
         private const string Unique = "Wox_Unique_Application_Mutex";
         public static MainWindow Window { get; private set; }
         public static PublicAPIInstance API { get; private set; }
+        private bool _saved;
 
         [STAThread]
         public static void Main()
@@ -71,5 +72,29 @@ namespace Wox
                 CommandArgsFactory.Execute(args);
             }
         }
+
+        private void OnExit(object sender, ExitEventArgs e)
+        {
+            Save();
+        }
+
+        private void OnSessionEnding(object sender, SessionEndingCancelEventArgs e)
+        {
+            Save();
+        }
+
+        private void Save()
+        {
+            // if sessionending is called, exit proverbially be called when log off / shutdown
+            // but if sessionending is not called, exit won't be called when log off / shutdown
+            if (!_saved)
+            {
+                var vm = (MainViewModel) Window.DataContext;
+                vm.Save();
+                PluginManager.Save();
+                ImageLoader.Save();
+                _saved = true;
+            }
+        }
     }
 }

Wox/ViewModel/MainViewModel.cs

@@ -18,7 +18,7 @@ using Wox.Storage;
 
 namespace Wox.ViewModel
 {
-    public class MainViewModel : BaseViewModel
+    public class MainViewModel : BaseViewModel, ISavable
     {
         #region Private Fields
 
@@ -81,7 +81,7 @@ namespace Wox.ViewModel
             InitializeKeyCommands();
         }
 
-        ~MainViewModel()
+        public void Save()
         {
             _settingsStorage.Save();
             _queryHistoryStorage.Save();