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/Src/VideoConfig.h

@@ -97,6 +97,7 @@ struct VideoConfig
 	bool bTexFmtOverlayEnable;
 	bool bTexFmtOverlayCenter;
 	bool bShowEFBCopyRegions;
+	bool bLogFPSToFile;
 	
 	// Render
 	bool bWireFrame;