Sortable: fix scroll jump on sort. Fixed: #9309 - Sortable: scroll jumps up if list is at the bottom of the page

After fix: <http://jsfiddle.net/pDevf/>

Root of the problem is in the order of how and when stuff is being created. placeholder is being created split millisecond after we assign absolute position to the placeholder. That is enough for browser to catch that the overall height of the window's been changed which leads to a scroll jump.