sendBackward method
Moves an object one step backward in z order.
Implementation
bool sendBackward(String id) {
final index = _objects.indexWhere((object) => object.id == id);
if (index <= 0) return false;
_remember();
final object = _objects.removeAt(index);
_objects.insert(index - 1, object);
render();
_emitChange();
return true;
}