Support other QtLocation map provider plugins in ondemandnavi

The ondemandnavi navigation application currently uses the Mapbox map provider plugin in QtLocation.  It has been pointed out that this is problematic in China, as Mapbox is inaccessible there.  It should be feasible to make the plugin configurable to allow selecting another one of the QtLocation plugins such OpenStreetMap.

I'm currently marking this as targeting Halibut, as the changes should hopefully be small enough to go into one of the point releases.  If not, it will be merged for Icefish instead.