We wanted to select an extruder based on wether or not it has support-material, so that the user doesn't have to think about selecting a support extruder any more and in most cases, can't forget anymore either. The formula present in fdmprinter to do that was not only an unreadable nightmare, but also very slow. We decided to pull most of that functionality into the settings-function machinery instead (but just a bit more generic so other properties can be selected, not just 'material_is_support_material').
done as part of finishing off CURA-10643
