BUG: multiple selections are lost when re-sorting

[Dirk Hohndel]

This used to work.

- Start Subsurface
- Select several dives inside one or more trip(s)
- change sort column (dives are still selected)
- change back to 'trip sort'
- only the 'selected_dive' is still selected

As it happens, that first successful change of sorf column (from trips
to one of the others) appears to be the ONLY one where we keep the selection.

/D