Move assetdir off of Graphics

It's only used in FileSystemUtils and never anywhere else, especially
not Graphics. Why is this on Graphics again?

It's now a static variable inside FileSystemUtils. It has also been
renamed to assetDir for consistency with saveDir and levelDir. Also,
it's a C string now, and is no longer an STL string.

desktop_version/src/Graphics.h

@@ -184,7 +184,6 @@ public:
 	bool onscreen(int t);
 
 	void reloadresources(void);
-	std::string assetdir;
 
 
 	void menuoffrender(void);