Some code getting data from OsmPlacemarkData instances currently
does a double-lookup with data.containsTagKey(x) & data.tagValue(x).
As OsmPlacemarkData already exposes begin and end iterators for the
tags, adding a findTag(const QString &key) to allow single-lookup
of a tag on conditional access to its value seems matching.
Resulting code trades readability (IMHO) for speed, but for a library this
option seems to make sense.