Add and use OsmPlacemarkData::findTag(const QString& key)

Summary:
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.

Reviewers: shentey, sanjibanb, nienhueser, #marble

Reviewed By: nienhueser, #marble

Differential Revision: https://phabricator.kde.org/D2684