Automatically load any WADs in the root of a zip/dir.

I've been dropping realm667 resource WADs into the root of a directory; it's zero-effort, avoids the fuss of moving all the parts to the right places, preserves the authorship information automatically, and is natively supported by ZDoom.  But not by SLADE!  So here you go.

The only other zip-supporting port SLADE knows about is Eternity, which sounds like it [supports embedded WADs as well](http://www.doomworld.com/vb/eternity/62087-eternity-engine-3-40-30-alfheim/).

Hmm, I did just realize that the WADs are supposed to be read [in alphabetical order](http://zdoom.org/wiki/PK3#How_to).  `ZipArchive` and `DirArchive` probably just read in disk order, right?  Is this addressed anywhere else or do you assume it's the author's problem if there are conflicts in the first place?  :)