document for modders
new trait Parachtuable that decides on the fate upon landing
new setting to allow unlucky paratroopers
@pchote's quest against legacy truth value definitions
merge ParachuteAttachment and Parachutable
make the parachute animation optional

instead of hard-coding default sprite sequences
make the parachute palettes configurable
add a customizable parachute shadow

closes #3335
make the fallrate customizable
make the parachute shadow sprite offset customizable