Small refactor of curtains
Small refactor of curtains

This commit ports Baystation12/Baystation12#8921.
Curtains now use SHOWER_OPEN_LAYER and SHOWER_CLOSED_LAYER instead of
manual layer definitions. Two new default shower curtains have been
added, Engineering and Security. They are not currently avalible on the
map due to the fact that Paradise's map, as of this commit, does not
have any showers anywhere but the dorms & engineering satellite.

ZomgPonies/Paradise64,ZomgPonies/Paradise64,ZomgPonies/Paradise64,ZomgPonies/Paradise64,ZomgPonies/Paradise64,ZomgPonies/Paradise64