clear-queue! function

Like win-clear!, except that it works in any view.
current-track, win-selected, track-* functions

current-track returns a track object which can be interrogated with
various track-* functions (e.g. track-artist).  win-selected returns a
track object as well if the current window is a track window.  In the
library artists/albums windows it returns a string (the selected
artist/album name).

With this it should be possible to write e.g. tag editing scripts which
call external tag editors using shell!.
<mpd/database.h> & <mpd/entity.h> functions
update! and rescan! functions
shell-sync!: return exit status
<mpd/playlist.h>/<mpd/directory.h> functions
Fix resize bug in library view

Call redraw-ui whenever the library data changes, so that the *current*
window size is known.