Always create grid menu so units are up-to-date.

Fixes: lp:1777519
* https://bugs.launchpad.net/kicad/+bug/1777519

common/tool/grid_menu.cpp

@@ -3,7 +3,7 @@
  *
  * Copyright (C) 2015 CERN
  * @author Maciej Suminski <maciej.suminski@cern.ch>
- * Copyright (C) 2015-2017 KiCad Developers, see AUTHORS.txt for contributors.
+ * Copyright (C) 2015-2018 KiCad Developers, see AUTHORS.txt for contributors.
  *
  * This program is free software; you can redistribute it and/or
  * modify it under the terms of the GNU General Public License
@@ -51,6 +51,13 @@ GRID_MENU::GRID_MENU( EDA_DRAW_FRAME* aParent ) : m_parent( aParent )
 }
 
 
+CONTEXT_MENU* GRID_MENU::Clone() const
+{
+    // Just return a fresh menu.  This keeps the units up-to-date.
+    return create();
+}
+
+
 OPT_TOOL_EVENT GRID_MENU::eventHandler( const wxMenuEvent& aEvent )
 {
     OPT_TOOL_EVENT event( ACTIONS::gridPreset.MakeEvent() );

include/tool/context_menu.h

@@ -142,7 +142,7 @@ public:
     /**
      * Creates a deep, recursive copy of this CONTEXT_MENU.
      */
-    CONTEXT_MENU* Clone() const;
+    virtual CONTEXT_MENU* Clone() const;
 
 protected:
     ///> Returns an instance of this class. It has to be overridden in inheriting classes.

include/tool/grid_menu.h

@@ -34,6 +34,8 @@ class GRID_MENU : public CONTEXT_MENU
 public:
     GRID_MENU( EDA_DRAW_FRAME* aParent );
 
+    CONTEXT_MENU* Clone() const override;
+
 private:
     CONTEXT_MENU* create() const override
     {