implement wxListBox::EnsureVisible() in wxGTK; add a test for it to the widgets sample

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_2_8_BRANCH@51659 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/gtk/listbox.h

@@ -74,6 +74,8 @@ public:
     virtual int GetSelection() const;
     virtual int GetSelections(wxArrayInt& aSelections) const;
 
+    virtual void EnsureVisible(int n);
+
     static wxVisualAttributes
     GetClassDefaultAttributes(wxWindowVariant variant = wxWINDOW_VARIANT_NORMAL);
 
@@ -119,6 +121,9 @@ protected:
 private:
     void Init(); //common construction
 
+    // common part of DoSetFirstItem() and EnsureVisible()
+    void DoScrollToCell(int n, float alignY, float alignX);
+
     DECLARE_DYNAMIC_CLASS(wxListBox)
 };