* Store empty setting too. Fixes toggle settings.
* Use proper IDs in toggle form fields
* Add a set_value() method to settings class.
* Fix: can't save a page with no prior setting.

If no $old_value exists, we can't merge it with the $new_value array, so just return $new_value.
* Fix: only update options for currently visible tab
* Add a method to detect and port data from older versions
* Document how to port data to the new version.