Implement wxListBox::EnsureVisible() for wxMSW.

Manually check whether the item is currently visible and scroll the listbox
just enough if it isn't.

Closes #3224.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@77653 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

interface/wx/listbox.h

@@ -285,12 +285,8 @@ public:
     /**
         Ensure that the item with the given index is currently shown.
 
-        Scroll the listbox if necessary.
-
-        This method is currently only implemented in wxGTK and wxOSX and does
-        nothing in other ports.
-
-        @see SetFirstItem()
+        This method scrolls the listbox only if necessary and doesn't do
+        anything if this item is already shown, unlike SetFirstItem().
      */
     virtual void EnsureVisible(int n);