sortlistmodel: Redo the way we store the items

sortlistmodel: Redo the way we store the items

We need to keep this data around for changes in future commits where we
make the sorting stable.

An important part of the new data handling is that the unsorted list
needs to always be dealt with before the sorted list - upon creation we
rely on the unsorted iter and upon destruction, the sorted sequence
frees the entry leaving the unsorted sequence pointer invalid.

This change does not do any behavioral changes.