MVVM refactoring for web search plugin, part 1

1. #486
2. fix #778 #763 #742
3. MVVM refactoring
4. remove IMultipleActionKeywords interface, use PluginManager directly

Plugins/Wox.Plugin.WebSearch/SettingsViewModel.cs

@@ -0,0 +1,22 @@
+using Wox.Infrastructure.Storage;
+
+namespace Wox.Plugin.WebSearch
+{
+    public class SettingsViewModel
+    {
+        private readonly JsonStrorage<Settings> _storage;
+
+        public SettingsViewModel()
+        {
+            _storage = new PluginJsonStorage<Settings>();
+            Settings = _storage.Load();
+        }
+
+        public Settings Settings { get; set; }
+
+        public void Save()
+        {
+            _storage.Save();
+        }
+    }
+}