[web/data] fix treeview regression (closes #3526466) Changeset 68cde7431c2c "[js] remove 3.9 bw compat (where apparently unused)" removed the use of form.callback from loadxhtml, which treeview relied on. Update to add a callback on the loadxhtml return value instead.

backout 8f3e963501e2, which is not ready yet

[pkg] also bump it there