merge with stable

worker: flush messages written by child processes before exit I found some child outputs were lost while testing the previous patch. Since os._exit() does nothing special, we need to do that explicitly.

worker: add basic test to ensure child processes are managed well This should catch the bug fixed by "worker: ignore meaningless exit status indication returned by os.waitpid()." Before, worker.py was untested since test repositories are relatively small.

color: rename '_styles' to '_defaultstyles' for clarity This should make it clear the dict is only used for new config. Extensions should not modify it directly anyway since we have 'extraloader' logic for loading '_styles' too.

color: move 'styles' definition on the 'ui' object Same logic as for '_terminfoparams'. The content depends on the config so it should be specific to each 'ui instance.

color: pass 'ui' to 'win32print' Same logic as before,'win32print' relies on some data we will move on the 'ui' object soon, we update the API beforehand for clarity.