Roll back the need to pass a Tree to the tree messages

[davep]

The following Tree messages:

• Tree.NodeCollapsed
• Tree.NodeExpanded
• Tree.NodeHighlighted
• Tree.NodeSelected
• DirectoryTree.FileSelected

all recently acquired an __init__ parameter that is the Tree they're related to; this was done to service the addition of control properties on all of those messages. That parameter isn't really necessary because each message knows about its node and the node knows its tree. So control can be written like this:

def control(self) -> Tree:
    return self.node.tree

[github-actions]

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