Don't keep references to cells not in the 'items' colltection

[pieter-v]

If some items (that are currently visible) are removed from the ‘items’ collection, then they are still referenced in the _cells property. So they are still in the DOM (display: none). If afterwards these removed items are destroyed (item.destroy) and a re-rendering (e.g.: change of screen size) occurs. Then this result in an error: Assertion Failed: Cannot call writableTag after the object is destroyed.

This pull request removes all cells that are not in the items collection.