Sorting titles has a structural defect

Nox the order of sorting in the Item class is page > column > position > level. In some cases the add_missing_titles() function adds a missing item to fil a gap but not in the correct position because the default position of missing item is (0,0)

Example with the pdf ETUDE D IMPACT.pdf where this situation can happend: page 58 / level 2 / origin fix page 61 / level 3 / origin missing page 61 / level 2 / origin fix page 61 / level 4 / origin fix

For the moment the hierarchical order has been changed into page > column > level > position