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; }

Wox.Core/UserSettings/PluginSettings.cs

@@ -1,10 +1,12 @@
 using System.Collections.Generic;
 using System.Linq;
+using PropertyChanged;
 using Wox.Core.Plugin;
 using Wox.Plugin;
 
 namespace Wox.Core.UserSettings
 {
+    [ImplementPropertyChanged]
     public class PluginsSettings
     {
         public string PythonDirectory { get; set; }

Wox.Core/UserSettings/Settings.cs

@@ -1,5 +1,6 @@
 using System;
 using System.Collections.Generic;
+using System.Collections.ObjectModel;
 using System.Drawing;
 using Newtonsoft.Json;
 using PropertyChanged;
@@ -31,7 +32,7 @@ namespace Wox.Core.UserSettings
         // Order defaults to 0 or -1, so 1 will let this property appear last
         [JsonProperty(Order = 1)]
         public PluginsSettings PluginSettings { get; set; } = new PluginsSettings();
-        public List<CustomPluginHotkey> CustomPluginHotkeys { get; set; } = new List<CustomPluginHotkey>();
+        public ObservableCollection<CustomPluginHotkey> CustomPluginHotkeys { get; set; } = new ObservableCollection<CustomPluginHotkey>();
 
         [Obsolete]
         public double Opacity { get; set; } = 1;