Added a CnC classic mod

The goal of this mod is to have original game rules as reverse engineered by @Nyerguds and converted to yaml by @psydev. It uses old artwork by Harisson and @pchote and re-adds the slightly bit-rotted GDI01 single-player prototype with minor fixes. Included are the unmerged engine changes from pull request #2614 and #2602 as well as the maps converted by @dan9550 #2573.