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

include/wx/msw/listbox.h

@@ -93,6 +93,8 @@ public:
     int HitTest(const wxPoint& pt) const { return DoHitTestList(pt); }
     int HitTest(wxCoord x, wxCoord y) const { return DoHitTestList(wxPoint(x, y)); }
 
+    virtual void EnsureVisible(int n);
+
     // ownerdrawn wxListBox and wxCheckListBox support
 #if wxUSE_OWNER_DRAWN
     // override base class virtuals