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

Wox.Infrastructure/Exception/WoxPluginException.cs

@@ -14,9 +14,5 @@
         {
             PluginName = pluginName;
         }
-
-        public WoxPluginException(string msg) : base(msg)
-        {
-        }
     }
 }