EKF: Treat combined sideslip and airspeed as a horizontal aiding source

[priseborough]

The combination of airspeed and sideslip fusion acts to constrain horizontal velocity drift with wind relative dead reckoning, however this was not being recognised in the code. This change treats the combination of airspeed and sideslip fusion as an equivalent horizontal velocity aiding source and ensures the solution status velocity_horiz flag will remain true if a fixed wing aircraft with airspeed and sideslip fusion loses GPS.

[priseborough]

This was tested in SITL using gazebo_plane with takeoff initiated using 'commander takeoff' and GPs failed once in loiter using 'failure gps on'. The EKF reported the loss of GPS, eg:

WARN [ecl/EKF] 589068000: GPS data stopped, using only EV, OF or air data

and continued to dead reckon with a steadily increasing reported position uncertainty for several minutes before the commander triggered a failsafe when the reported horizontal position uncertainty exceeded 45m = 2.5 x COM_POS_FS_EPH (default of 15).

WARN [commander] Failsafe enabled: no global position