Comment and error handling for rename.
Small refactoring.

  Just moving stuff around, comments.
Heavy refactoring, moving completely load, rename and edit out of action.c
Moved private declaration directly in the appropriate function in t_renamer.c.
t_error is now in yaml.c
Ensure to save when needed.
Removed bad ref to t_error.h in t_yaml.c
Ensure to output in the current locale.

  * print was printing in UTF-8 in all cases. We ensure to use the
    locale defined by the env. print is the only action where we output
    tags so it's OK for now to do it just before output.

  * Input convertion is currently performed by the backend. Every
    backend does it except the old (and crappy) ID3v1.1. It's still not
    OK we need to have the same internal representation.
iconv stuff moved to t_toolkit.
Convert the argument input from user to UTF-8 in t_action.c

  That way much of the actions works with a different locale than UTF-8.
Cosmetics.
Always output and parse YAML in UTF-8.

  YAML is always in either UTF-8 or UTF-16.