Fixes regression from commit e96779de.

Finally restores an important feature that has been broken for several
months when the PlayerThread started working with Song copies instead
of pointers to the Queue's Song instances (commit e96779de).

This is the beginning of multi-player support.  There will be support
for multiple Partition objects in one MPD process.

Move the configuration lookup to Main.cxx.

For simpler seeking within current song.

Reduce header dependencies.

Only used there.

Sorts remaining songs by priority.  This can be used for the
much-demanded "queue feature".

Allocate a player_control object where needed, and pass it around.
Each "client" object is associated with a "player_control" instance.

This prepares multi-player support.

No more CD player emulation.  The current behaviour of "previous" is
difficult for a client to predict, because it does not definitely know
the current position within the song.  If a client wants to restart
the current song, it can always send "playid".

Renamed all playlist functions to non-CamelCase.

Those were only wrappers for playlist_state_{save,restore}().  Since
sf_callbacks has been removed, we can call the latter functions
directly.

To determine whether to rewind the current song or to go to the
previous song, use a GTimer instead of manually diffing time(NULL).

Consume mode removes each song played

When single mode is enabled, after current song it stops playback,
or it replay same song if repeat mode is activated.

The smartstop feature is a way to tell mpd to stop playing after
current song.
This patche provides:
- 'state' command returns 'smartstop' state (1 or 0)
- 'smartstop' can activate or not the smartstop state
- when song is terminated, mpd stops playing and smartstop is set to 0

The move command now accepts a range for the first argument, in the same
form as other range commands, e.g. move 15:17 3.  The first song in the
range is placed at the destination position. Note that as with other
range commands, the range is inclusive on the left only; this example
would move only songs 15 and 16, not 17.

[mk: fixed signed/unsigned warnings; use G_MAXUINT instead of
UINT_MAX]

This updates the copyright header to all be the same, which is
pretty much an update of where to mail request for a copy of the GPL
and the years of the MPD project. This also puts all committers under
'The Music Player Project' umbrella. These entries should go
individually in the AUTHORS file, for consistancy.

This command shuffles a range of songs.

In random mode, this patch allows clients to see the "next song" in
the queue.

Pass constant playlist objects to functions which do not modify it.