Water Stairs

-added upward and downward water stairs, so in the future we can have water bases.
-can't dive, probably due to the keybinding being the same as going down.
Probably "water_slope_down" and "channel down" or somesuch?  It's more likely that these'll be slopes rather than regular stairways.

(That said, it would let the stairs flood in the Strange Temple... so yeah.)