macosx: changed the data appearance in the info panel to the following behaviour...

macosx: changed the data appearance in the info panel to the following behaviour: if playlist is collapsed, we show data about the currently playing item. if the playlist is shown, we show data about the current selection in the list, regardless of the current item.

Empty selections in the playlist are still not allowed, as this would lead to some trouble I don't want to solve for this old panel (to be fixed once the dudiak branch is merged)