qml: do not immediately show placeholder artwork as soon as an item added to the playlist

Often times items get preparsed very quickly, so loading the placeholder image does not look good and wastes resources. If the item can not get preparsed for whatever reason in duration_humanMoment, then it removes the preparsed guard and after that the placeholder is shown.

If item is preparsed quickly but the item does not have an artwork, the placeholder is shown as well.

Request review @chub.