ImageSequenceExpander called Expander constructor before getPath method
could return meaningful value to look up the config from
seq_config_dict.
ImageExpander has no associated config therefore editing config items is
prohibited. It is achieved by vetoing the EVT_TREE_BEGIN_LABEL_EDIT
event. All expanders without meaning usage of config object shall
implement a getPath method that returns None. It is used in the
EVT_TREE_BEGIN_LABEL_EDIT event handler.