Add curtains to vehicle doors

This also enables the use of multiple openables on a single tile
add door curtains to default vehicles
Fix checking if outside in vehicle::open()
Merge branch 'veh-door-curtains' of https://github.com/dar-/Cataclysm-DDA into dar--veh-door-curtains
Formatting fix for broken build.

I messed up that translation tag, this fixes it.
Merge pull request #6565 from Rivet-the-Zombie/fix-fix

Formatting fix for broken build.
Rivet's paren fix
Merge branch 'master' of https://github.com/CleverRaven/Cataclysm-DDA