Improve alarm zone rendering, detection and audio.

This relates to issue #21.

- Fix the rendering of the alarm zones using the new improved draw_blob_gl().
- Fix the generation and detection of alarms.

What is still missing is a way to disable the alarm, instead of it only sounding once. I think the simplest way is to pop up a
confirmation dialog that a bogey has been detected. Until this is confirmed we should keep playing the sound.
Improve alarm zones

- Add a dialog that shows the alarm, and when the next sound will be generated. If this new dialog is confirmed or closed the alarm
  will be silent until there are no bogeys and fresh bogeys enter the alarm zone.
- Add a fallback sound -- a simple bell sound (wxBell()).
- Render the zone relative to the boat, not north. Still to check whether the alarm zones actually match properly with the rendered
  zone.
Small cosmetics
Merge branch 'master' of https://github.com/canboat/BR24radar_pi