sink.text.pretty: update logging macros

Change the logging macros to be more in sync with how it's done in other
component classes of the project

Add the `self_comp` and `log_level` fields to the pretty_component
structure, and change the BT_COMP_LOG_SELF_COMP and BT_LOG_OUTPUT_LEVEL
to assume there's always a pointer to such a structure named `pretty`.

Change-Id: I6292f3c9b6b7f238e335cfcc90259a1f0047313d
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/3405
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Tested-by: jenkins <jenkins@lttng.org>