mpdmenu: Factor out thumbnail generation.

mpdmenu: Factor out thumbnail generation.

Prep work for making use of MPD albumart, move all calls to the
thumbnailer into a single function, and adapt the menus to call
the function once to generate an entire list of cover art.

Drop the use of the --image option in the thumbnailer by poking at
the cover art directly, since this will no longer work with MPD
albumart.