Make PositionSource recover when backend is available

Currently applications need to recreate PositionSource objects in qml when backends has been unavailable. If PositionSource would recover when backend is available, application code will be much cleaner.

From documentation
PositionSource.ClosedError - The remote positioning backend closed the connection, which happens for example in case the user is switching location services to off. This object becomes invalid and should be deleted. A new source can be declared later on to check whether the positioning backend is up again.