awesome/mpd_widget: use ping command to sync state

awesome/mpd_widget: use ping command to sync state

The ping command does nothing other than return OK, but it will be sent
in order of the `send()` call, i.e. after any previous invocation of
`send()` and since the server will return strings in order, this
guarantees that the previous `status` and `currentsong` command have
been evaluated and contain data.