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.