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/core/private.h

@@ -609,6 +609,7 @@ public:
     // display
 
     virtual void            ListScrollTo( unsigned int n ) = 0;
+    virtual int             ListGetTopItem() const = 0;
     virtual void            UpdateLine( unsigned int n, wxListWidgetColumn* col = NULL ) = 0;
     virtual void            UpdateLineToEnd( unsigned int n) = 0;