Add a ShowContextMenu action

Allow users without physical MenuKey to open the context menu

Change-Id: Id886a5614e26d614cba6ceb3b08a9df6148e9655
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>

src/plugins/texteditor/texteditor.h

@@ -401,6 +401,8 @@ public:
     virtual bool selectBlockDown();
     void selectWordUnderCursor();
 
+    void showContextMenu();
+
     void moveLineUp();
     void moveLineDown();