Override DoExpand() in wxQt wxDataViewCtrl stub

This code doesn't work (and probably doesn't compile) anyhow, but at
least try to do the right thing in it and override DoExpand() instead of
adding a virtual Expand() hiding the non-virtual version in the base
class.

src/qt/dataview.cpp

@@ -267,7 +267,7 @@ wxRect wxDataViewCtrl::GetItemRect( const wxDataViewItem &item,
     return wxRect();
 }
 
-void wxDataViewCtrl::Expand( const wxDataViewItem & item )
+void wxDataViewCtrl::DoExpand( const wxDataViewItem & item )
 {
 }