Store history in application state directory

[nbraud]

Currently, history_path defaults to ~~/memo-history.log, ~~/ being mpv's config directory.
That should probably be ~~state/memo-history.log, which uses the application state directory: ${XDG_STATE_HOME}/mpv on various unices.

For more information about the distinction, see the XDG Base Directory specification.

[nbraud]

I'll be happy to submit a PR for this... just not at midnight 😉