promoting POIs from higher zooms

[rinigus]

Would be great to postprocess the generated database and promote some of the POIs from higher zooms to populate lower zoom levels if those are empty. That would allow to keep sane number of POIs on overview levels, but not leave them empty. For that, one just has to select (randomly?) a POI at a level lower (or if that is empty, even lower) and move it to a level above.