(Image credit: Valve) It took almost 20 years, but Team Fortress 2, the team-based FPS that changed a great many lives when it debuted in 2007, now has an SDK that Valve says will enable creators "to build completely new games based on TF2."The Team Fortress 2 SDK is actually a "massive update" to the Source SDK that adds all of the client and server game code, which opens the doors to pretty much anything. "Unlike the Steam Workshop or local content mods, this SDK gives mod makers the ability t...