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/ContainerTree.py

@@ -0,0 +1,15 @@
+# Copyright (c) 2019 Ultimaker B.V.
+# Cura is released under the terms of the LGPLv3 or higher.
+
+from cura.Machines.MachineNode import MachineNode
+
+from typing import Dict
+
+##  This class contains a look-up tree for which containers are available at
+#   which stages of configuration.
+#
+#   The tree starts at the machine definitions. For every distinct definition
+#   there will be one machine node here.
+class ContainerTree:
+    def __init__(self) -> None:
+        self.machines = {}  # type: Dict[str, MachineNode] # Mapping from definition ID to machine nodes.