Fix orientation & caging, add indicators & G Meter

Changes:

1. Sensor orientation now completes when sensors are mounted in any orientation.
   Previously when sensors were mounted "upside down" relative to my prototype board the orientation was rejected.
2. Only one cage process at a time is allowed.
   Previously if a user pressed "Cage" a second time while the algorithm was still calibrating from the first press,
   a second calibration would take place immediately, making it seem like it would never complete.
3. AHRS status lights were added to show whether the AHRS algorithm is using GPS, Attitude sensors (MPU9250), and/or
   Altitude sensors (BMP280), and whether it is currently Calibrating and/or Logging (which can eat up disk space quickly).
   It is especially important for the user to be able to see the calibration process occurring, so that it is apparent why the AI isn't responding.
4. A G Meter is shown on the GPS/AHRS page.
5. Magnetic Heading, Turn Rate, Slip/Skid Angle, and G Load are shown in text on the GPS/AHRS page.
   Magnetic Heading will not be useful until the magnetometer is calibrated.
   I may add an auto-calibration routine to a future release so that it will self-calibrate over the course of several flights, and/or add a manual calibration routine.
6. The slip/skid rectangle on the attitude indicator works now.
   For now, it will only indicate correctly if the sensor is level in coordinated flight.
   In the future, auto-calibration will do this for you automatically.
7. A few labels were changed in the UI to make meanings clearer to the casual user.
8. Deeper changes were made to the MPU9250 driver to improve its efficiency.
   One of these changes involved removing a rudimentary accelerometer calibration.
   As a result, when on the desktop, the AHRS will not quite indicate level--it will indicate the actual orientation of the sensor.
   Even fairly rigidly mounted boards may indicate a degree or two of pitch or bank when sitting on a level desk.
   For now, you can regard this as an assist in manually leveling your stratux on your glareshield.
   An upcoming release will include a proper calibration, which is necessary to provide better pitch estimation.