Tree doesn't post NodeExpanded when a node is expanded from code

[davep]

Noticed while starting work on #2456, Tree will only post a NodeExpanded message when a node is expanded under user interaction. If a node is expanded from code, no message is sent. This has a knock-on effect on the DirectoryTree where it used NodeExapnded to know when it should lazy-load the content of a directory in the tree.

Correcting this is necessary to implementing #2456, but adding this as an issue in its own right as it's a fairly fundamental change to Tree (albeit not a breaking one) and worth highlighting in its own right.

This will in effect expand on the work in #1644 (and so follows on from the requirement first laid out in #1430).

[davep]

Note that collapsing a node also has the same issue in respect to `Tree.NodeCollapsed.

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