Introduce Qt.labs.location QtLocationLabs singleton type

This singleton is meant to offer tech-preview map-related API.
It starts with a mapObjectsAt invokable, that can be used to probe
MapObjects at a specific coordinate of a map.
Reference implementation for Q*ObjectQSG, based on QGeoShape::contains,
included.

Change-Id: Ief692eb5a43115ca02d4642c82023d1b2e217400
Reviewed-by: Alex Blasche <alexander.blasche@qt.io>