Add "Set Rel" for directory bookmarks

see
http://pulkomandy.tk/projects/GrafX2/ticket/45
It is disabled under AmigaOS, AROS and MorphOS

src/io.h

@@ -195,5 +195,9 @@ int Remove_path(const char * path);
 /// Remove the directory
 int Remove_directory(const char * path);
 
+///
+/// Calculate relative path
+const char * Calculate_relative_path(const char * ref_path, const char * path);
+
 /** @}*/
 #endif