Also, add an example for the date picker

Moved from Cards to Tiles and made the printing
of the dates nicer by using some code from a Dart SDK example:
https://github.com/dart-lang/sdk/blob/master/samples-dev/swarm/swarm_ui_lib/util/DateUtils.dart

I also built a UserData class to help keep saving/sorting
consistent as well as fixed the sort order to have most
recent at the top.

@abarth