Support for zip archives with a single top-level folder

[argv]

Popular code-hosting sites like GitHub and Bitbucket can generate zip archives of the current state of a project, but ZDoom cannot currently load them because of their structure: they contain a single top-level folder, and the game data files (decorate, loadacs, sprites, etc) are all inside of that folder. ZDoom expects the game data files to be at the top level in the archive, not inside a folder.

If ZDoom were taught how to load this kind of archive, then it would be trivial for people to play a mod that's hosted on one of these sites: just download the latest zip and load it into the game.

[Graf Zahl]

Added.

[Caligari87]

Woohoo! Thank you Graf!