small updates to quick display

For operation by people with full sized aviation experience in the US we need a few small updates to the quick display options:
- verticalspeed in feet/minute when using mph for units. Maybe "vertspd_fpm" ?
- glide ratio (horizontal speed from GLOBAL_POSITION_INT, divided by vertical speed from GLOBAL_POSITION_INT)

This is for a mission we're doing on Monday :-)

Start by locating the quick display options and the handling of GLOBAL_POSITION_INT in Mission Planner. Add a feet-per-minute vertical-speed option for mph units and a glide-ratio display using horizontal speed divided by vertical speed; done means both values appear correctly in the quick display.

Written by the indexing model from the issue text.