No drag-drop merge animation for piles with a cover

[flekschas]

Commit or branch: master

Observed behavior

1. Piles with a cover do not animate when one pile is dropped onto another pile
2. The previews move animation starts from the beginning every time
3. After merging the aggregation flickers as its updated

Expected behavior

1. Piles with a cover should animate when one pile is dropped onto another pile
2. The previews move animation shouldn't be reset for item previews that are already displayed
3. After merging the aggregation should transition from the old into the new aggregation

Steps to reproduce