Added a new QGeoServiceProviderFactoryV2 to load plugins offering
a navigation manager engine.

To be able to still load plugins compiled against Qt 5.10, both factories
are considered when loading plugins, using the pointer to the V2 only
when accessing the newly exposed functionality.

Change-Id: I8e5e868737c77142e77caaacef278686565928df
Reviewed-by: Alex Blasche <alexander.blasche@qt.io>