Rename SimpleListModel to WorldListModel and make it specific for
WorldClocks

Because it is now specific, it listens to changes to the WorldModel and
broadcasts changes to all registered ListDataListeners.
Make TIME_FORMATTER and DATE_FORMATTER public constants
Expand the WorldClock class

It now uses a clock for timing and provides getters for its fields.
Add WorldClockRenderer which displays the WorldClocks properly in the
view
Let WorldModel update the time

WorldModel now listens for PropertyChangeEvents from the ClockModel and
updates all WorldClocks if necessary.
Add action to add new WorldClocks

AddAction adds new WorldClocks to the list.