Fixes #165.

This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

Fix for the problem where order with priorities gets out of whack in case it's
reordered by SetRandom() while another song is currently playing.
What happens is, if some song is already playing and you have set some
priorities before switching on the random mode, and then turn the mode on, the
original code swaps position of the first song in the order (i.e., the highest
priority song) with current, so that current is 0 (which it should be). The
problem is, the "original" first song then goes to the place "current" song was
after reordering, wherever that is, instead of going after the "current" song.
This patch fixes the issue.
Also the fix makes MoveOrder() public, because why shouldn't it be, anyway.  It
certainly makes more sense than just having SwapOrders() public for some
reason.
Signed-off-by: Eugene Baklanov <miltenfiremage@gmail.com>

Works around a build failure on Solaris because annoyingly, Solaris
reserves the name "queue".  This rename was pending anyway.

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.

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