Add wxDataViewCtrl::ExpandChildren()

This convenient method allows to expand all children of the item
recursively.

This is directly supported by both native implementations, so it makes
sense to have this in wxDataViewCtrl itself, even if it could be also
(less efficiently) implemented in the user code.

src/qt/dataview.cpp

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