AltitudeHold: tune the amount of tilt compensation

The existing code would still drop during forward flight,
because of the non-linearities of open loop ESCs. This
allows tuning the amount of tilt compensation.
AltitudeHold: make compensation amount a uint16

Being able to only go to the power 2.5 was insufficient to
maintain forward flight at steeper angles.
Merge pull request #1171 from peabody124/tune_tilt_comp

AltitudeHold: tune the amount of tilt compensation