a GPS can now be attached after startup

the old mechanism wasted most of the input bytes, and chewed a lot of
CPU, making it impractical to do GPS detection for a GPS attached
after startup

The new code is async, and detects a GPS by looking for a fully formed
packet with the right checksum for each GPS type

this prevents non-manual channels changing value away from the trim
value on startup

remove unused enable_out_ch() and enable k_manual channels. This
ensures k_manual channels used for bottle drop are initialised
correctly to trim output

fixed off by one error

this makes it possible to have a reverse throttle in ArduPlane

Changes include using AP_PeriodicProcess and calling imu.init even in HIL mode

a few UI translations

this was broken in 2.61

this makes it easier to diagnose elevon mixing issues

thanks Alexey!

allow disabling of the use of the barometer for vertical acceleration
compensation

this drives the speed of the 50Hz loop by the number of samples
accumulated in the IMU. This should give much more consistent timing
in DCM.

Thanks to Randy for introducing this scheme in ArduCopter!

this fixes using GPS_PROTOCOL to specify a specific GPS with a GPS
that takes a few updates before it works (eg. needing baud rate
changes).

This makes it easier to use an APM1-1280 with more features enabled

this allows for a larger amount of steering control at low speeds
without causing osciallation after takeoff