Architecturally-simpler fix for grid context menu unit changes.

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

common/tool/grid_menu.cpp

@@ -51,13 +51,6 @@ 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() );
@@ -70,9 +63,17 @@ OPT_TOOL_EVENT GRID_MENU::eventHandler( const wxMenuEvent& aEvent )
 
 void GRID_MENU::update()
 {
+    BASE_SCREEN*  screen = m_parent->GetScreen();
+    int           currentId = screen->GetGridCmdId();
+    wxArrayString gridsList;
+
+    screen->BuildGridsChoiceList( gridsList, g_UserUnit != INCHES );
+
     for( unsigned int i = 0; i < GetMenuItemCount(); ++i )
-        Check( ID_POPUP_GRID_SELECT + 1 + i, false );
+    {
+        int menuId = ID_POPUP_GRID_SELECT + 1 + i;
 
-    // Check the current grid size
-    Check( m_parent->GetScreen()->GetGridCmdId(), true );
+        SetLabel( menuId, gridsList[i] );      // Refresh label in case units have changed
+        Check( menuId, menuId == currentId );  // Refresh checkmark
+    }
 }

include/tool/context_menu.h

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

include/tool/grid_menu.h

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