queue/Queue: add methods MoveOrderBefore() and MoveOrderAfter()

src/queue/Queue.cxx

@@ -215,6 +215,24 @@ Queue::MoveOrder(unsigned from_order, unsigned to_order) noexcept
 	return to_order;
 }
 
+unsigned
+Queue::MoveOrderBefore(unsigned from_order, unsigned to_order) noexcept
+{
+	/* if "from_order" comes before "to_order", then the new
+	   position is "to_order-1"; otherwise the "to_order" song is
+	   moved one ahead */
+	return MoveOrder(from_order, to_order - (from_order < to_order));
+}
+
+unsigned
+Queue::MoveOrderAfter(unsigned from_order, unsigned to_order) noexcept
+{
+	/* if "from_order" comes after "to_order", then the new
+	   position is "to_order+1"; otherwise the "to_order" song is
+	   moved one back */
+	return MoveOrder(from_order, to_order + (from_order > to_order));
+}
+
 void
 Queue::DeletePosition(unsigned position) noexcept
 {

src/queue/Queue.hxx

@@ -290,6 +290,24 @@ struct Queue {
 	unsigned MoveOrder(unsigned from_order, unsigned to_order) noexcept;
 
 	/**
+	 * Moves a song to a new position in the "order" list before
+	 * the given one.
+	 *
+	 * @return the new order number of the given "from" song
+	 */
+	unsigned MoveOrderBefore(unsigned from_order,
+				 unsigned to_order) noexcept;
+
+	/**
+	 * Moves a song to a new position in the "order" list after
+	 * the given one.
+	 *
+	 * @return the new order number of the given "from" song
+	 */
+	unsigned MoveOrderAfter(unsigned from_order,
+				unsigned to_order) noexcept;
+
+	/**
 	 * Moves a song to a new position.
 	 */
 	void MovePostion(unsigned from, unsigned to) noexcept;