Support position accuracy in QNmeaPositionInfoSource.

Extract the HDOP and VDOP from the NMEA stream and calculate the twice
the distance root mean square (2DRMS) error based on the user provided
User equivalent range error value.

[ChangeLog][QtPositioning][QNmeaPositionInfoSource] Added support for
reporting position accuracy.

Change-Id: I59e5f8c32070fa96ae4d0bd02a18f38663920e05
Reviewed-by: Alex Blasche <alexander.blasche@digia.com>