ATK mapping for summary element should be ROLE_TOGGLE_BUTTON #142

The current ATK mapping for the summary element is ROLE_PUSH_BUTTON. But the summary element behaves more like a toggle button and thus should be mapped accordingly.

BTW: The summary and details elements look and behave much like GtkExpander. And GtkExpander's accessible object is a ROLE_TOGGLE_BUTTON; not a ROLE_PUSH_BUTTON.