sink.text.pretty: use bt_common_g_string_append and bt_common_g_string_append_c

sink.text.pretty: use bt_common_g_string_append and bt_common_g_string_append_c

Use our own inline implementation of g_string_append and g_string_append_c
to improve text pretty-printing speed.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: I39254ae912cb9bd227c68519552b4b610c15755f
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1506
CI-Build: Philippe Proulx <eeppeliteloop@gmail.com>
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>