color: drop the 'colorui' class Now that all logics formally bared by 'colorui' have been moved to the main ui class, that class is empty and can be dropped. As a nice side effect we can get rid of the baroque Initialization associated to it. There was much rejoicing.

color: move 'write-err' logic to the core ui class This is similar to what we needed for 'write', we move the logic from the extension to the core class. Beside the dispatch to 'win32print', we just apply label to the argument.

ui: extract low level part of 'write_err' in its own method Same as for 'write' and '_write' we needs it available for some of the color logic.