Implement a simple benchmarking mode which logs FPS to a file

Very useful to compare performance between two builds, check the impact of
a configuration option, etc. FPS log is stored in User/Logs/fps.txt and is
reset each time you launch a game. Only enabled if you check the "Log FPS
to file" option in your graphics settings.

Could be improved a bit: currently logs only every 1s (so you can't really
see small variations), maybe output more infos to the fps.txt like
average/stddev (but Excel/Libreoffice/Google Docs can compute that easily
too).

Source/Core/VideoCommon/CMakeLists.txt

@@ -6,6 +6,7 @@ set(SRCS	Src/BPFunctions.cpp
 			Src/DLCache.cpp
 			Src/Debugger.cpp
 			Src/Fifo.cpp
+			Src/FPSCounter.cpp
 			Src/FramebufferManagerBase.cpp
 			Src/HiresTextures.cpp
 			Src/ImageWrite.cpp