feature: Only reload specific train file on clone track node

[TheGamerzs]

Allow LoadTrainTrackTreeNodes to be called when cloning the track node in a background thread. Adds all nodes to the UI at once, instead of each time; improves UI responsiveness/time.

I can't remember the speed increase, but iir, it was like 3-5s to 500-850ms "lag"/render lock.

Changing the track nodes to a Linked List may be better so we can insert new nodes at x. Anything that forces a WinForms update seems to render lock

I think WinForms updates will be necessary from x -> end, unless the node data is updated under the hood - But node type will still needed to be updated x -> end as its displayed in the UI