Add bt_plugin_set object

Add bt_plugin_set object

It is unusual in the Babeltrace API to return a NULL-terminated array of
objects which must be individually put by the user in addition to
calling free() on the returned array. This is what was returned by
bt_plugin_create_all_from_file(), bt_plugin_create_all_from_dir(),
and bt_plugin_create_all_from_static().

Instead, create a bt_plugin_set object of which the only purpose is to
contain plugins. A plugin set has a reference count and is not more
awkward to use than any other standard Babeltrace object.

Tests and the CLI are updated accordingly.

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