Clean up unnecessary exports and add static keywords

This patch cleans up unnecessary exports from header files (there were
only a few), as well as adds the static keyword to all symbols that
aren't exported and are specific to a file. This helps the linker out in
not doing any unnecessary work, speeding it up and avoiding silent
symbol conflicts (otherwise two symbols with the same name (and
type/signature in C++) would quietly resolve as okay by the linker).

desktop_version/src/GraphicsResources.cpp

@@ -24,7 +24,7 @@ extern "C"
 	);
 }
 
-SDL_Surface* LoadImage(const char *filename, bool noBlend = true, bool noAlpha = false)
+static SDL_Surface* LoadImage(const char *filename, bool noBlend = true, bool noAlpha = false)
 {
 	//Temporary storage for the image that's loaded
 	SDL_Surface* loadedImage = NULL;