GCS/Input: use THROTTLE_NEUTRAL_FRACTION for all places
GCS/Input: make sure neutral always at THROTTLE_NEUTRAL_FRACTION

The visualization of throttle during the direction check looked
strange because the input range for throttle was asymmetrical.
GCS/Input: keep all channel neutral positions mid range
GCS/Input: fix throttle neutral when inverting channel
GCS/Input: formatting fixes requested in review
Merge pull request #1357 from peabody124/input_wiz_fixes

More UI fixes for the input wizard