Set notification iterator methods to the component class

Set notification iterator methods to the component class

This follows the spirit of the previous refactorings by assigning
notification iterator methods to the component class instead of setting
them during their initialization method.

Conceptually here, a (source and filter) component class defines one,
and only one notification iterator class. Therefore the concept of an
iterator "class" is hidden to the user here, since there's no
one-to-many relationship. In any OO language, an iterator class would be
a class nested under a component class.

Source and filter component classes are created with two mandatory
iterator methods: get and next.

The initialization, destroy, and seek time iterator methods are
optional.

New functions:

* bt_component_class_source_set_notification_iterator_init_method()
* bt_component_class_source_set_notification_iterator_destroy_method()
* bt_component_class_source_set_notification_iterator_seek_time_method()
* bt_component_class_filter_set_notification_iterator_init_method()
* bt_component_class_filter_set_notification_iterator_destroy_method()
* bt_component_class_filter_set_notification_iterator_seek_time_method()

The plugin development interface (babeltrace/plugin/plugin-dev.h) is
updated accordingly.

Tests and existing plugins are updated accordingly.

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