Add wxDataViewCtrl::GetTopItem() and GetCountPerPage()

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

Closes #17498.

include/wx/dataview.h

@@ -699,6 +699,9 @@ public:
     wxDataViewItem GetCurrentItem() const;
     void SetCurrentItem(const wxDataViewItem& item);
 
+    virtual wxDataViewItem GetTopItem() const { return wxDataViewItem(0); }
+    virtual int GetCountPerPage() const { return wxNOT_FOUND; }
+
     // Currently focused column of the current item or NULL if no column has focus
     virtual wxDataViewColumn *GetCurrentColumn() const = 0;