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>