naming of functions in panels_allowed_layouts

Some more thoughts on the naming issue:
- 'set_allowed': What is being set is already said by the class name, so we don't need the 'allowed' part. Also, the 'set' part is a bit misleading: it resembles 'save' and sounds like this function sets or saves data, while it does only retrieve a form. Following the Drupal quasi standards, I'd propose 'edit' or 'page' or 'edit_page'.
- 'sync_with_available'. Again, it is obvious what is being synced with, so 'sync' would be just fine.
- 'api_save' might be a bit confusing, as it sounds like the API would be saved, whatever that would mean. I don't have a better idea though, as we are using 'save' for custom save functions.
- 'list_layouts' could be prepended with an underscore as it is a quasi-private function (and will be a real private one in D7).