qml: drop indicator should wait until drop operation is completed in PlaylistListView (regression !3801)

Drop indicator should not hide until drop operation is completed.

In !3531 (merged), it was made possible to resolve items early so that the drop operation is completed synchronously. However in !3801 (merged), it was reverted to asynchronous while this not being taken into account for the drop indicator which uses containsDrag property of the drop area. If drop operation is done asynchronously, containsDrag already evaluates false when the drop handling is not done yet.