adapted to new tourism-value "apartment"

The "tourism=chalet" tag was split into two, adding a new tag
"tourism=apartment". (see
https://wiki.openstreetmap.org/wiki/Tag:tourism%3Dapartment).
I use the same icon still as for chalet, but lowered zoom level from 17
to 18, since apartment are expected to be in more dense areas than
chalets.