Remove two obsolete sprite-handling externs.
Remove space object externs.

Go through accessor functions.
Return nullptr for kNoShip.
Merge branch 'extern' into develop
Fix key conflict flashing.

_flashed_on is used for "flashed on at the moment", not "flashing
enabled".
Initialize _next_flash with int.

It's not a double.
Merge branch 'flash' into develop