config: use the new '_unset' value for 'configbool' This should let 'configbool' delegate all special processing of the default config value to the main 'config' method. The default value for bool (False) is still enforced in this method if no other default were passed.

config: explicitly track the use of the standard default value We introduce a small object used to detect that no specific default value has been passed to 'ui.config'. We need this explicit special value since "None" is a valid and common default value. The end goal here is to make progress on a centralised and explicit declaration of the available config option. A first good usecase for this are "default" value. Before starting looking further down this alley we needs to rework the handling of default value in the 'ui' object to have all configxyz methods going through the same logic. This is the first changeset on this trek.