Introduce map objects reference implementation

This patch introduces a reference implementation for
the QGeoMapObject subclasses in the locationlabs module.
If this module is built, all current plugins, with
the exception of mapboxgl, will also be able to render
map objects.

The current reference implementation of map objects
is not optimized, but it can be useful for both testing
and feature parity (so that switching between plugins
will give the same result).

Change-Id: I830ebb3813f219e42c085f450952a2b4327361cd
Reviewed-by: Alex Blasche <alexander.blasche@qt.io>