Return an array of all the embedded cells of an element. If all you need is id's of all the embedded cells, use element.get('embeds'). If opt.deep is true, all the deeply embedded cells will be returned. The order in which the cells are returned depends on the search algorithm used. By default, Depth-first search (DFS) algorithm is used. If opt.breadthFirst is true, the Breadth-first search algorithm will be used instead.