text.pretty: use clock class with highest priority to print the timestamp

text.pretty: use clock class with highest priority to print the timestamp

Also, if there's no clock class in the event notification's clock class
priority map, then do not print any timestamp.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>