Refactoring CustomizedPluginConfig

1. Reorder the sequence of initialization of UserSettings.
2. Use dictionary for CustomizedPluginConfigs, so code logic like
`.FirstOrDefault(o => o.ID == id);` are removed
3. part of #389

Wox.Core/UserSettings/CustomizedPluginConfig.cs

@@ -1,9 +1,8 @@
-using System;
-using System.Collections.Generic;
+using System.Collections.Generic;
 
 namespace Wox.Core.UserSettings
 {
-    [Serializable]
+
     public class CustomizedPluginConfig
     {
         public string ID { get; set; }