Plane: added GROUND_STEER_ALT and GROUND_STEER_DPS

this uses the new AP_SteerController steering controller developed for
the rover code to allow for ground steering of planes
Plane: give manual steering when still

useful for servo testing
Plane: split out the channel stick mixing

makes code a bit clearer
Plane: if pilot gives aileron input then disable ground steering

otherwise it would be very tricky to do a low pass!
APM_Control: change minspeed on steering controller to 1.0