feat: drop as sibling to open folder

[cha0s]

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Somewhat related to #140 and #144

Instead of having to fuss about internal nodes and hovering, it seem to me that the real solution is just to allow dropping nodes as siblings of open folders (and not always as children).

This is a fix I'm testing, though you may identify some issue with it. It seems to do the right thing. I thought I'd start a conversation! :)