Gamers still hoping against hope for a true follow-up to Half-Life 2 (after so, so many years ) may remember last year’s flurry of speculation that Half-Life 3 was finally nearing completion . Amid all those rumors, though, we never got any official sign from anyone inside Valve that a Half-Life project was currently in development. That changed this week thanks to an apparent slip-up during a panel at GameSoundCon , an industry conference devoted to "the art, technology, and business of game audio."…