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.

terrain_nav: add setting callback

- Ensure the planner is updated when a setting changes.

terrain_nav: set setting callback in constructor

- Remove duplicated setting update code.

Signed-off-by: Rhys Mainwaring <rhys.mainwaring@me.com>

Author: srmainwaring