Move the block for resizing the preview, applying an item effect + border to the slot where the preview has been received.

This reduces the blocking of the GUI for the following usecase:
- folder with 1000 items
- ~30 items are visible

Old approach:
- Show the 30 items as soon as possible.
- Receive preview and add it to the queue. This is done asynchronously 970 times (no UI blocking).
- Iterate through 970 items, resize, apply item effect + border and apply the preview to the model (-> blocking of UI thread for more than 3 seconds).

New approach:
- Show the 30 items as soon as possible.
- Receive preview, resize it, apply item effect + border and add it to the queue. This is done asynchronously 970 times (no UI blocking).
- Iterate through 970 items and apply the preview to the model (-> UI thread is blocked only very short).