qml: set view cursor to busy until the drop operation is completed

This is mainly relevant for the main playlist, as medialibrary playlist already does this since !5781 (merged).

One might ask if this would conflict with !5781 (merged), because medialibrary playlist also uses ListViewExt. It is safe to say that it is not the case, because medialibrary playlist adjusts the control cursor while this one adjusts the view cursor. This one complements !5781 (merged) in the sense that now the busy cursor would be shown as early as the drop operation starts (this merge request), and ends as late as medialibrary transaction is done (current case).