When the random mode is toggled, MPD did not clear the queue.  Because
of this, MPD continued with the next (random or non-random) song
according to the previous mode.  Clear the queued song to fix that.

Merged duplicate code from queueNextSongInPlaylist().

Added several comments.

Move everything which belongs together into one common struct.  This
simplifies the implementation of several queue operations.

Attempt to untie the playlist.c knot: moved the playlist storage code
to queue.c, struct queue.

Check for current>=0, not queued>=0.

The function moveSongInPlaylist() attempted to read the position of
the current song, even if it was -1.  Check that first.  The same bug
was in shufflePlaylist().

Pass const pointers where no writes are performed.

The null plugin synchronizes the playback so it will happen in real
time.  This patch adds a configuration option which disables this: the
playback will then be as fast as possible.  This can be useful to
profile MPD.

Free memory in the finish() method to make valgrind happy.

Renamed functions and variables.

It is possible that playlist.current is reset before the TAG event
handler playlist_tag_event() is called.  Convert the assertion into a
run-time check.

Break from the loop instead of returning the function.  This calls
player_stop_decoder(), which in turn emits the PLAYLIST event.  This
allows the playlist to re-start the player.

The function player_seek_decoder() starts the decoder, but does not
check the return value of player_wait_for_decoder().

Make player_wait_for_decoder() and play_chunk() return a bool instead
of 0/-1.

Don't attempt to restart the player if it was stopped, but there were
still songs left on the playlist.  This looks like it has been a
workaround for a bug which has been fixed long time ago.

Don't assign the result of getPlayerError() to an integer.

Use g_free() consistently with g_malloc().  Don't clear the pointers
after freeing them.

The player_thread loop requests the next song from the playlist as
soon as the decoder finishes the song which is currently being played.
This is superfluous, and can lead to synchronization errors and wrong
results.  The playlist already knows when the player starts playing
the next song (player_wait_for_decoder() triggers the PLAYLIST event),
and will then trigger the scheduler to provide the next song.

Simplify some code by using config_get_positive(), instead of doing
manual parsing and validation each time.

This convenience function parses a configuration value into a positive
integer.  It aborts if parsing fails.

player_wait_for_decoder() emits the PLAYLIST event on success, remove
the duplicate PLAYLIST event in its caller.

The "TAG" event is emitted by the player thread when the current
song's tag has changed.  Split this event from "PLAYLIST" and make it
a separate callback, which is more efficient.

The "sticker" command allows clients to query or manipulate the
sticker database.  This patch implements the sub-commands "get" and
"set"; more will follow soon (enumeration), as well as extended
"lsinfo" / "playlistinfo" versions.

When a song is deleted from the database, remove its sticker, too.

What's still missing is some sort of garbage collector after a fresh
database create (--create-db).

This is a wrapper for the backend sticker library.  It provides
several glue functions which take a song object instead of type/uri.

"Stickers" are pieces of information attached to existing MPD objects
(e.g. song files, directories, albums).  Clients can create arbitrary
name/value pairs.  MPD itself does not assume any special meaning in
them.

The plugin queries build-time configuration variables, and should
include config.h.

Minimize header dependencies.

Eliminate some more getBlockParam() invocations.

This replaces lots of getBlockParam() invocations.

Don't modify the configured string.

Determine the audio output name with audio_output_get_name() instead
of obtaining the name from the configuration again.

Use config_get_path() instead in mapper.c.

The g_free() function includes a NULL check.  We don't have to do it
twice.

This causes a segmentation fault...