Add QGeoPolygon to QtPositioning

This patch introduces a new QGeoShape, QGeoPolygon, together
with helper functions in the location singleton (QtPositioning.*)
to create and convert geopolygons from QML.

[ChangeLog][QtPositioning][QGeoPolygon] Added QGeoPolygon shape.

Change-Id: I111c576d7428f2a953f0459d16c25eea7ab2bd7c
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@qt.io>