This function shuffles the last song of a range.  This is used by
addSongToPlaylist().

The function shuffles the virtual order of songs, but does not move
them physically.  This is used in random mode.

The new function replaces playlist.c's randomizeOrder() function,
which was aware of playlist.current and playlist.queued.  The latter
is always -1 anyway, and the former as preserved by the caller, by
converting playlist.current to a position, and then back to an order
number.

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.