appDisplay: Move from instanceof-testing to polymorphic duck typing

Instead of having _compareItems, _getItemId, etc. on the view to
pull out info about items, use the AppIcon / FolderIcon items we
create as a place to track this additional info. We now require
that these items have a '.id' property for deduplication, and a
'.name' property to sort by.

https://bugzilla.gnome.org/show_bug.cgi?id=722117