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.
Circling diameter and full circle time infobox (enhancement #2939)

Adds a Cirle diameter infobox which displays estimated circle diameter and
full circle flight time. Useful for evaluating best thermalling mode with a
glider at different wing loading.
Repetitive warning sound for airspace warnings dialog (enhancement #2952)

Adds repetitive warning sound when approaching or inside the prohibited airspace and
airspace warnings dialog is shown.
This is useful if you miss initial warning sound in sometimes noisy glider
cockpit and are close to airspace.
Warning sound is played with increased frequency as you are closer to airspace.
Enabling of this feature is configurable in Airspace configuration page by
checkbox "Repetitive Sound".
Improved Thermal assistant appearance (enhancement #2368, #2954)

Enchances visibility of thermal assistance information to be able to see it in
shaking glider with a corner of an eye in gauge mode.
Following improvements done:
- Thickened thermal polygon border line
- Thickened and darkened 0 lift line for clearer reference where is lift/sink
- Changed thin dashed outter circle to display MC setting if it is below max lift
and added text value to it
- Changed max_lift text to display actual max value instead of rounded up range
- Thickened and lengthened thermal advisor line with bigger ball at the end
to indicate in which direction the center of the thermal is.
changed from WARNING_GLIDE to WARNING_GLIDE_OR_FILTER