ui_factory.note does not support unicode

[Vincent Ladeuil]

Separated from bug #563997 which was selftest specific.

TextUIFactory doesn't handle unciode for show_error(), show_warning() and show_message() (the later relying on note()).

How this should be handled is unclear. My understanding is that we currently rely on make_output_stream() but that seems targeted at stdout while show_error and show_warning write to stderr.

We may want different ways to find the relevant encoding.

Also note that currently only prompt() takes care of encoding its output by using osutils.get_terminal_encoding().

But osutils.get_terminal_encoding() is not cached. It should may be stay that way but in that case we should make sure the streams used by the ui objects have an 'encoding' attribute.

On the overall, we may also want to better unify with the trace module.