Displaced items were moving unnecessarily

Displaced items were moving unnecessarily

They should only move if they actually change from the last set
position, and not if they are simply changing from their current item
position, as that is wrong during an animation.

This also cleans up some code for resetting the transition data.

Task-number: QTBUG-24586
Change-Id: I0a6635903975ebc40d5cf8398b943a9de92d4493
Reviewed-by: Martin Jones <martin.jones@nokia.com>