core (Cagou): add debug log for widget visibility + ignore_missing: - debug log are now use to show which widget are made (in)visible, and to show the list of currently visible widgets - "ignore_missing" can be used in _removeVisibleWidget when we can expect that a widget is missing in visible widgets list.

[DEFAULT]
bridge = pb
log_level = debug
log_fmt = #[%%(levelname)s][%%(name)s] %%(message)s

[cagou]
no_certificate_validation = true
log_level = debug
log_fmt = [%%(levelname)s][%%(name)s] %%(message)s