_add_spread_elements_from_idml(): Create a list of elements to add and then add them to the destination Spread.
Add IDMLPackage.get_layer_id_by_name().
Add IDMLPackage.test_get_spread_elements_by_layer().
Add elements from same layer. Fix regression test.
Add Spread elements from the same layer first to keep the z-axis position.
Exclude Guide from Spread elements added from the same layer.
Bump to v0.91.7.