actions: introduce action group

This patch introduces action groups as a new kind of action.

When creating a trigger, it is only possible to attach a single action.
Action groups allow users to attach more than one action.

A group is created using lttng_action_group_create.  Actions are added
to it using lttng_action_group_add_action.  The group can then be used
in a trigger, like any other action.

The operations required to be implemented by actions (serialize,
create_from_buffer, validate) are implemented by executing the operation
on all elements.

Current limitations are:

- To avoid any cycle, it is not possible to add a group inside a group.

Signed-off-by: Jonathan Rajotte <jonathan.rajotte-julien@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Change-Id: I2ae6aed21d9a6b45510d37a435773b1bd7d76528