Turned out to be a quite easy fix. I thought the issue was that we
couldn't identify the TreeStoreElem when (re)creating its TreeElement item
correctly, because for non-ID elements that would be index dependent (=
bad for drag & drop). Turns out that we're actually allowed to store
custom data within the TreeStoreElem, the thing is just that it gets
stored as ID pointer (highly ugly and highly misleading).
Anyway, seems to work now so I won't complain too much :)
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