There was quite a lot of duplicate code for loading DetachedSong
objects, with different semantics for "securely" loading local files.

Detach the Song class completely from the public API, only to be used
by SimpleDatabase and the update thread.

Make the DetachedSong(Song) conversion constructor private.  Everybody
should use map_song_detach() which will take over more
responsibilities soon.

Don't create an intermediate Song instance when all we want is a
DetachedSong.

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.

Don't use IgnoreError() when there's an overload that does not try to
give us one.

Fixes purging deleted songs from the queue after update.  Fixes
regression from commit e96779de.

Fix for a major memory leak.

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

Replaces GLib's GError.

Use the C++ API.

Allow the plugin to allocate the GetSong() return value.

Rewrite of the pc_get_error_message() function, now using a GError
object instead of the complicated "errored_song" attribute.

Moving songs using either 'move' or 'moveid' to position -1 (after the
current song) would fail for a song which is just before the current
song.
This patch corrects the check to see if the current song is in the range
to be moved. Since the range is from `start` up to `end` (exclusive) the
check was incorrect, but is now fixed.

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.

After we've been hit by Large File Support problems several times in
the past week (which only occur on 32 bit platforms, which I don't
have), this is yet another attempt to fix the issue.