Add utils.dummy component class, move plugins/trimmer to plugins/utils

Add utils.dummy component class, move plugins/trimmer to plugins/utils

This patch adds a plugins/utils directory where the utils plugin sources
are. plugins/trimmer is moved to this new directory because the trimmer
component class is part of the utils plugin.

Also in the utils plugin is the dummy sink component class. This sink
does absolutely nothing with its notifications. It is the equivalent of
the BT 1.x dummy format. At consume time, it gets one notification per
input notification iterator and returns
BT_NOTIFICATION_ITERATOR_STATUS_END when all the iterators are at the
end.

The legacy `-o dummy` option is not handled specifically in this commit.
You can use the dummy sink as usual:

    babeltrace /path/to/trace -o utils.dummy

The dummy component class ignores its initialization parameters.

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