Return an str with the current engine version. If key doesn't doesn't exist, assume 'Source', otherwise invalid -- assume next-gen 'Source 2'.

Buried underneath layers of code in Valve's Source Filmmaker script files there are more than 60 references to "Source 2." That's part one to this news story; it appears Valve's next-gen game engine, the first full-fledged new version of Source since 2004, is coming soon. The sequel piece of news, Source Engine 2: Electric Boogaloo, is really just speculation but absolutely worth noting: if Source Engine 2 is on the way, can Half-Life 3 be far behind?Courtesy of the folks at the ValveTime blog , we learn of the Source 2 mentions in Filmmaker, including this most explicit line: Valve launched Source in November 2004 with Counter-Strike: Source, but it really showed off what the engine is capable of later that same month with the launch of Half-Life 2. So does Source 2 herald the coming of Half-Life 3? Valve certainly isn't talking, but it does give reason to hope.