We want to make sure that this tree is constructed during start-up after all containers have been registered, so we call getInstance() there once. If you need the tree before that, the tree will not yet have been filled and you won't get complete information, so you'd need to listen for updates.
The singleton is there so you don't need to go via CuraApplication.
Contributes to issue CURA-6600.
This sets up a few new classes, subclasses of ContainerNode.
This is intended to simplify the current structure in the QualityManager.
Contributes to issue CURA-6600.
