Refactoring ContextMenu

1. Remove ItemDropEvent
2. Remove ShowContextMenus from API
3. Fix context menu item can't be opened ( #535 ), bug introduced from PR #494 (commit 45dbb50)
4. Move open result command and load context menu command back to
MainViewModel
5. unify load context menu logic
6. other performance enhancement and potential bug fixed

Wox.Plugin/IPublicAPI.cs

@@ -15,12 +15,6 @@ namespace Wox.Plugin
         /// <param name="results"></param>
         void PushResults(Query query, PluginMetadata plugin, List<Result> results);
 
-        /// <summary>
-        /// Show context menu with giving results
-        /// </summary>
-        /// <param name="results"></param>
-        void ShowContextMenu(PluginMetadata plugin, List<Result> results);
-
         /// <summary>
         /// Change Wox query
         /// </summary>
@@ -115,11 +109,5 @@ namespace Wox.Plugin
         /// if you want to hook something like Ctrl+R, you should use this event
         /// </summary>
         event WoxGlobalKeyboardEventHandler GlobalKeyboardEvent;
-
-        /// <summary>
-        /// Fired after drop to result item of current plugin 
-        /// </summary>
-        /// todo: ResultItem -> Result
-        event ResultItemDropEventHandler ResultItemDropEvent;
     }
 }