Introduce QGeoMapObject::geoShape property

In this way MapObjects become a little closer to MapItems,
and this property can be used to, for example, center the map
on one specific object or object group.

Fixes: QTBUG-69640
Change-Id: I4dbead9e670bf5d1eeaccb2bd09d956b8de7da87
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@qt.io>