chat: avatar improvments: - avatar source is not set anymore in kv but on MessageWidget init, so None value can be handled correclty - Message widget add itself to mess_data before parsing kv, so call to properties in kv can update the widget correctly (specially important for embedded bridge)

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

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