Add clock class priority map object

This new object is a simple map of clock classes to their priorities
against the other clock classes of the same map. See the header
file's documentation for more information (clock-class-priority-map.h).

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