Various mpd updates and improvements.

Various mpd updates and improvements.

This changeset primarily adds inserting coverart into the balloon-help
buffer, but it also cleans up the code and fixes a couple of tiny bugs
and prevents bytecompiler warnings.

	* mpd.el (mpd-start-connection): Add #'mpd-clear-variables to
	mpd-before-variables-update-hook.
	(mpd-disconnect): Remove it here.
	(define-mpd-command): Improve docstring.
	(mpd-state): New.  Returns the value of `**mpd-var-state*'.
	(mpd-stopped-p, mpd-paused-p): Use it.
	(mpd-volume): New. Returns the current volume as int.
	(mpd-muted-p): New.  Return t when volume is zero.
	(mpd-volume-up): Use #'mpd-volume, don't run any hooks.
	(mpd-volume-down): Ditto.
	(mpd-volume-mute): Define with `mpd-define-command' to have hooks
	run.  Use #'mpd-volume.
	(mpd-volume-min): Use #'mpd-volume, improve docstring.
	(mpd-songpos): Prevent bytecompiler warnings.
	(mpd-seek): Ditto.
	(mpd-seek-forward, mpd-seek-backward): Add docstring.
	(mpd-file): New. Return a mpd track filename.
	(mpd-cover-file): New. Returns a possible coverart filename.
	(mpd-has-cover-p): New.  Returns t when coverart exists
	for the current track.
	(mpd-scale-cover): New.  Scales coverart image.
	(mpd-update-cover): Rewrite with #'mpd-file, #'mpd-cover-file,
	#'mpd-has-cover-p, #'mpd-scale-cover
	(mpd-clear-variables): New.  Sets Title, Artist, Album, Genre, and
	Date variables to nil when they don't exist in the metainfo.
	(mpd-info): Prevent bytecompiler warnings.  Allow space for
	coverart in balloon when necessary.
	(mpd-balloon-cover): New.  Inserts coverart into mpd balloon.
	(balloon-help-display-help): Advised to insert coverart.

Signed-off-by: Steve Youngs <steve@sxemacs.org>