ExpandedListItem animate nicely

List item delegates currently pop out directly followed by the focus highlight resizing with an animation on top. The result looks terrible closing.

Instead we can animate the height. The expandable part is wrapped in an item that relays the implicit height of the expandable contents when expanded, but not otherwise. This allows us to have a separate timeline for the expanded item visibility and a simple animation on the root item takes care of the rest.

Contents are expanded and fade in sync, which makes for (IMHO) a clean look.

Video is qmlscene --slow-animations. It's not actually this slow.