Add WindowOrganizer and WindowManager

These classes will handle the window management, but are currently not
connected with the remaining code base. A WindowManager is responsible
for one window. The WindowOrganizer organizes all WindowManagers.
Add TimeModel

The TimeModel is now the parent model of ClockModel and WorldModel.
Make TimeModel the parent of StopModel
Add start method to TimeController

The TimeController does not start anymore on creation, but after the
start method is called.
Fix bug: Open menu is empty
Change over to the new window management

Add a WindowManager class for each window and change StartClock to set
up a WindowOrganizer using these WindowManagers.

The ClockView class has been changed to create the JButtons, that open
the other windows, using the Actions provided by the WindowOrganizer.