terrain_nav: add alt waypoint generator and settings dialog

- Add WP generator that uses MAV_CMD_NAV_LOITER_TO_ALT
- Update window size
- Use wxsettings module for dialog.
- Ensure dialog retrieves settings from correct thread.
- Add setting callback, ensure the planner is updated when a setting changes.
- Remove duplicated setting update code.
- Add home, start loiter and goal loiter waypoints
  - Add waypoint at home position
  - Add start and goal loiter mission items.
  - Calculate start and goal loiter directions.
- Remove waypoint at loiter exit
  - Extra waypoint on loiter radius results in a full turn as heading is not achieved.
- Correct simple waypoint count

Author: srmainwaring