Introduce `bp-notifications-actions.php` to handle single notification status toggling.

* Include new actions file in loader.
* Tweak link output functions to make it moderately easier to debug the URL.
* Introduce `bp_notifications_mark_notification()` to help with marking single notifications.
* Fix `bp_notifications_delete_notification()` to work with new array argument pattern, introduced previously.
* It's alive!

See #5148.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7543 cdf35c40-ae34-48e0-9cc9-0c9da1808c22