adding 2nd data validation list after load

[JanMarvin]

When saving, wb$dataValidationsLst is moved to ext. Upon loading it is not moved to dataValidationsLst but remains in extLst. Hence we need to check if creating another entry in dataValidationsLst and saving again properly merges or creates broken xlsx output. Preferably the list is not silently moved from one entry to another, but remains at a single spot. E.g. load it, move the node around and remove the node in the extLst.

This will be broken. Has one extLst, but two "<ext xmlns:x14 ...>" entries.

 wb2$add_data_validation("Sheet 3", col = 2, rows = 2:31, type = "list",
                         value = "'Sheet 4'!$A$1:$A$10")

[JanMarvin]

This is one of the bugs that Excel cannot recover from. E.g. it complains but gives up and you cannot see whatever was in the file.