Fix for correct handling of relative filenames.

Now relative filenames that are not below the .blend file
in the directory hierarchy can be used.

CAUTION: 
  The relative filenames are not updated if the blend file
  is moved to another dir or saved into another dir (save as)
  We will rely on the smartness of the users for this.

my first official commit - thanks Ton!

source/blender/blenlib/BLI_blenlib.h

@@ -125,7 +125,7 @@ void BLI_splitdirstring(char *di,char *fi);
 	 */
 int BLI_convertstringcode(char *path, char *basepath, int framenum);
 
-void BLI_makestringcode(char *fromfile, char *str);
+void BLI_makestringcode(const char *relfile, char *file);
 
 	/**
 	 * Change every @a from in @a string into @a to. The