Use Java Preferences API

At the moment the user can specify some preferences using the CaGe.ini file. Maybe this should be switched to Java Preferences API:

the CaGe.ini file can still be used for default values in case the backing store is not available, or CaGe is ran for the first time.
we can provide a GUI within CaGe to edit the preferences.
Preferences can even be changed while running the program

CaGe-graph / CaGe