Make the header only contain the public interface and nothing from the
implementation. All functions get a handle to their self object, like the img_
and tns_ and win_ functions. All necessary data (file path) is also passed as
an argument, so that no extern redeclarations are needed.

Make arl_setup_dir() private, it's not called outside the module.

Make arl_handle() return true if the file has changed, so that the reloading of
the file can be done by the caller.