Flight: rename flight modes to acro and leveling

Previous commits introduced RATE and ATTITUDE as the name
for flight modes, but this creates ambiguity for the per-axis
settings versus the overall flight mode.

Now the modes are named ACRO and LEVELING, which is consistent
with some of the other software platforms out there. Again, the
original STABILIZED{1,2,3} are still available.
GCS Setup Wizard: use LEVELING and ACRO as default settings