Thermalling straightening notifier sound (enhancement #2955)

Adds a sound notification when straightening of circle is needed during thermalling.
Sound notification is played around ~1.5 seconds before thermal center becomes
ahead of glider's path during circling.
When entering the circle glider must make at least a complete circle in either
direction before sound notification starts to be played. This is needed for
algorithm to acquire most complete lift distribution in circle to provide accurate
straightening notifications.