qt: fix unable to drag a large collection of media

When Drag.active is unset, the FSM goes into inactive state even though the input items are yet to be parsed.

Unlike loading done and loading failed states, inactive state does not have an handling mechanism when the input items are ready.

We should not fail the drag early if the input items can not be collected until the drag operation finishes. Ideally, this should be a synchronous operation, but it can be asynchronously handled like this as well because we can invalidate the drag if the user starts a new drag operation before the previous operation is completed.

Close #28762 (closed). Request review @chub.