Heartbeat API: throttle down when the window looses focus or when the user is inactive, always send 'screen_id', change the interval settings to 'fast' (5sec), 'standard' (15sec) and 'slow' (60sec), the interval can be changed from PHP, see #23216

git-svn-id: https://core.svn.wordpress.org/trunk@23382 1a063a9b-81f0-0310-95a4-ce76da25c4cd