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.Infrastructure/Storage/ISavable.cs

@@ -0,0 +1,11 @@
+namespace Wox.Infrastructure.Storage
+{
+    /// <summary>
+    /// Save plugin settings/cache, 
+    /// todo should be merged into a abstract class intead of seperate interface
+    /// </summary>
+    public interface ISavable
+    {
+        void Save();
+    }
+}