Make ContainerTree singleton but construct in run() of application

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.

cura/Machines/ContainerTree.py

@@ -14,6 +14,14 @@ from typing import Dict
 #   The tree starts at the machine definitions. For every distinct definition
 #   there will be one machine node here.
 class ContainerTree:
+    __instance = None
+
+    @classmethod
+    def getInstance(cls):
+        if cls.__instance is None:
+            cls.__instance = ContainerTree()
+        return cls.__instance
+
     def __init__(self) -> None:
         self.machines = {}  # type: Dict[str, MachineNode] # Mapping from definition ID to machine nodes.
         container_registry = ContainerRegistry.getInstance()