Do not remove DOM element when dismissing an item, but move it to back

We now move the DOM item to the back and update the state of the
ScrollingEngine.

When dismissing we now delete the item from the fakeData. For doing so
an id is added to all items.

It is still not possible to dismiss items after the physical index
length as items with transform are not moved properly. This is not
a regression and will be fixed seperately.