Add wxListBox::GetTopItem() and GetCountPerPage()

Implement these methods for all the major ports, add them to the widgets
sample and documentation.

Closes #17189.

include/wx/osx/listbox.h

@@ -108,6 +108,8 @@ public:
 
     virtual void EnsureVisible(int n) wxOVERRIDE;
 
+    virtual int GetTopItem() const wxOVERRIDE;
+
     virtual wxVisualAttributes GetDefaultAttributes() const wxOVERRIDE
     {
         return GetClassDefaultAttributes(GetWindowVariant());