Add css class to article/block/component when on screen

[lc-thomasberger]

As mentioned here

This will make it easy to add css based animations when a Element is scrolled into view.

"_onScreen": {
    "_isEnabled": true,
    "_classes": "bounceOutLeft",
    "_options": {
        "_percentFromTop": 25,
        ....
    }
}

new syntax:

{
        "_id": "b-05",
        "_parentId": "a-05",
        "_type": "block",
        "_classes": "animated",
        "title": "b-05",
        "displayTitle": "",
        "body": "",
        "_trackingId": 0,
        "_onScreen": {
            "_isEnabled": true,
            "_classes": "wobble",
            "_percentInviewVertical": 50
        }
}

Will need to decide if we should only add classes or also remove them ...

[andrewhosgood]

Maybe have a look at http://imakewebthings.com/waypoints/

[lc-thomasberger]

as for now add on-screen and off-screen classes
add to core and test if this causes serious performance issues

[moloko]

PR: #1320