A priority should be a volatile thing: it should schedule a song for
playing back once, but it should not affect the next time the queue
gets played.

Empty currently, but code will be added song.

From now on, struct Song will be used by the database only, and
DetachedSong will be used by everybody else.  DetachedSong is easier
to use, but Song has lower overhead.

Fixes regression from commit e96779de.

Leave that to the caller, allowing it to modify multiple items at a
time.

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).

Prepare to migrate away from GLib.  Currently, we're still using GLib
as a backend.

Move the configuration lookup to Main.cxx.

This commit reimplements the core of the "single" mode.  Instead of
doing the detection in the playlist code from the outside, it is moved
to the player thread, which gets a new option called "border_pause".
It will now pause playback exactly at the beginning of the new song,
making the feature more reliable.

Now that the player thread knows what will happen, it can suppress
cross-fading.

Fixes mantis tickets 0003055 and 0003166.

Make sure the player "owns" the next_song object, so nobody else can
free it.

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.

The check was meant to fix an assertion failure, but it was the wrong
way around.  This broke cross-fading most of the time.