Add inactivity notification

Add inactivity notification

The new inactivity notification indicates that there's no activity to be
expected on the iterator pointing to it until a given time. You specify
the given time with one or more clock values which are associated to
clock classes. Those clock classes must be mapped to a priority in the
notification's clock class priority map.

This notification is similar to the event notification, except that it
does not contain any context or payload. Its purpose is to make the
graph more efficient with live sources which know that a given iterator
won't deliver any nofication which "occur" before a given time. For
example, a muxer can add this notification to its internal heap, just
like it adds event notifications. Most sinks should ignore this
notification.

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