Add wxDataViewCtrl::GetTopItem() and GetCountPerPage()

Add methods doing the same thing for wxDataViewCtrl as the existing wxListBox
methods.

Closes #17498.

include/wx/gtk/dataview.h

@@ -169,6 +169,9 @@ public:
 
     virtual wxDataViewColumn *GetCurrentColumn() const wxOVERRIDE;
 
+    virtual wxDataViewItem GetTopItem() const wxOVERRIDE;
+    virtual int GetCountPerPage() const wxOVERRIDE;
+
     static wxVisualAttributes
     GetClassDefaultAttributes(wxWindowVariant variant = wxWINDOW_VARIANT_NORMAL);