mpdmenu: Retrieve cover art from MPD.

Recent versions of MPD support retrieving cover art, which means we
(mostly) don't need to have the music directory mounted to poke around
in it.  Augment mpdexec with support for binary transfers in order to
retrieve this cover art, and implement a new thumbnailer script which
makes use of this functionality.

Mostly, because the function to launch geeqie in the scans directory
still requires poking in the music dir.  But all the menu images should
be displayed just from the connection to MPD now.

As a bonus, the new script is quite a bit faster than the old one, too.