FlightMode: add rate, attitude, and virtual bar

These are explicit modes that will be used like Stabilized#,
but have the benefit of having a clear meaning for things
like GCS. Currently not implemented.
Flight: connect up new flight modes

These are the following hardcoded combinations of channel
settings:

 - RATE:       rate, rate, axis lock
 - ATTITUDE:   attitude, attitude, axis lock
 - VIRTUALBAR: virtualbar, virtualbar, axis lock

The custom modes STABILIZED{1,2,3} are still available like
previously.
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
Merge pull request #1188 from peabody124/flight_modes

Flight modes