Introduce `BP_Notifications_Notification` methods for handling LIMIT and ORDER BY MySQL to act as the foundation for customizing sorting and pagination arguments. Also flip default notification order from ASC to DESC for a top-down view. See #5148.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7549 cdf35c40-ae34-48e0-9cc9-0c9da1808c22