We can reuse our duplicateMaterial function again but in a simpler way. Also finding the preferred material is simpler with our container tree.
However there seems to be a problem with finding the preferred material; it's not finding generic_pla for UM3 and AA0.4 anyway, and then falls back on a random material. This needs to be fixed in the variant node class.
Contributes to issue CURA-6600.