Fix undercarriage icon staying up while wheels being lowered

When pressing F7 with the undercarriage up, to lower it, the icon flashes to a down arrow and then goes back to an up arrow again until the undercarriage is fully deployed. Fixed by setting the icon to down while it is being lowered too, which required querying the transition status.
Autopilot undercarriage sound fix

When the autopilot sets the undercarriage down, it calls SetWheelState(true) in every frame, which causes the undercarriage sound to be triggered repeatedly until it is fully deployed. Fixed by not considering SetWheelState to succeed if the transition value doesn't change.
Cleaner way to do the same
Proofread Spanish translations
Merge remote-tracking branch 'pgimeno/undercarriage'
replace NULL with plain old 0 as the C++ gods intended
pull a bunch of game constants out of libs.h and give them proper definitions
SIZE_FMT is not a build option
remove DEG/RAD constants, only use the conversion functions
move PIONEER_DATA_DIR down to the only place where its actually used
remove pi_load_lua and CurrentDirectory
remove RefList, its unused
remove BezierCurve, also unused
initialise rmb flag
silence faction debug
Merge remote-tracking branch 'robn/minor-cleanups'
Factions.cpp: Silence unused variable warning