Override GetMainWindowOfCompositeControl() in wxGrid subwindows.

This is necessary for HasFocus() to work correctly for them since the changes
of r69788.

Closes #13686.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@69861 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/generic/private/grid.h

@@ -301,6 +301,8 @@ public:
         m_owner = owner;
     }
 
+    virtual wxWindow *GetMainWindowOfCompositeControl() { return m_owner; }
+
     virtual bool AcceptsFocus() const { return false; }
 
     wxGrid *GetOwner() { return m_owner; }