AC_PosControl: vel controller becomes stabilized

Velocity controller updated to interpret the velocity requests as
desired velocities (i.e. feed forward).  These are then used to update
the target position and also added to the target velocity.
Also renamed the set_desired_velocity() function to
set_desired_velocity_xy() to make clear only lat and lon axis are
updated.
AC_WPNav: integrate set_desired_velocity_xy function name change
Copter: guided mode sets desired velocity instead of target velocity