Databinding for hotkey tab

part of #660

Wox.Core/UserSettings/PluginHotkey.cs

@@ -1,7 +1,9 @@
 using System;
+using PropertyChanged;
 
 namespace Wox.Core.UserSettings
 {
+    [ImplementPropertyChanged]
     public class CustomPluginHotkey
     {
         public string Hotkey { get; set; }