Rework container tree structure

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.

cura/Machines/MachineNode.py

@@ -0,0 +1,19 @@
+# Copyright (c) 2019 Ultimaker B.V.
+# Cura is released under the terms of the LGPLv3 or higher.
+
+from typing import TYPE_CHECKING
+
+from cura.Machines.ContainerNode import ContainerNode
+from cura.Machines.ContainerTree import ContainerTree
+from cura.Machines.VariantNode import VariantNode
+
+if TYPE_CHECKING:
+    from typing import Dict
+
+##  This class represents a machine in the container tree.
+#
+#   The subnodes of these nodes are variants.
+class MachineNode(ContainerNode):
+    def __init__(self, container_id: str) -> None:
+        super().__init__(container_id, None)
+        self.variants = {}  # type: Dict[str, VariantNode] # mapping variant IDs to their nodes.