Make ClockModel use a Clock and include the current date

ClockModel is now based on a Clock, which allows for easier testing. It
also keeps track of the current date and fires PropertyChangeEvents on
change.
Clean up AbstractModel

Rename propertyChanged to firePropertyChange as in
PropertyChangeSupport.
Split ClockView into the container ClockView and the time displaying
component TimeView

ClockView allows to add other components easily to the gui. TimeView is
the former ClockView and has received a new name to better describe that
it only displays the time.
Add DateView to display date
Fix bug: Clock displays UTC and not local time

The default constructor of ClockModel now creates a clock based on the
default time zone and not based on UTC.
Make gui look nicer

Introduce a border around the gui to make things look better. Also the
size of the frame is adjusted.