* Remove remaining hard del()'s in our code.
* Replace deleted() and gcDestroyed with QDELETED macro.
* Fix some strange calls directly to Del() (capital D)
* Make Destroy() return qdel hints.
* Make a few of the Destroy()'s more comprehensive in cleaning up references.
* In edited Destroy() procs, converted to use qdel_null and qdel_null_list macros when possible for pretty code.
* Removed unused variable `sleevecard`
* Don't infinately accumulate underlays!
* Ease up on the New() and Destroy(), make sure they are a top level object (loc is a turf)
* The open space darkness overlay is always the same, make it a global.
* Instead of open spaces simply showing a static image of the turf below, it shows the turf, all objects, and even moving mobs.
* Controller updates the view to floor below fairly quickly; mobs update in real time.
* Open space can be examined to see how deep it is.
* Mobs are examinable even when on floor below, using a special "zshadow" mob.
* Changes from Eris: Heavily re-written to handle layering, special cases, rewrote controller etc. Shadow mobs don't break entering turfs, etc.
* Update to properly layer the objects from multiple "below" floors in the case of tripple-stacked or more z levels.
* Added constant defines for the planes in use.
* Added open_space.dmi with blank_open icon state for open space's darkening overlay.
