Update Preference Model

[Adam Jocksch <ajocksch at redhat dot com>]

Ok, over the last couple weeks Sami and myself have come up with what we 
think is a fairly robust and extensible model for the preferences in 
Frysk. Types of preferences are modeled by a class (eg. IntPreference, 
ColorPreference), through which you can retrieve the current value, set 
a new value, and add a listener to be notified when the preference 
changes. These preferences are then organized into preference groups, 
which have a name and can return an iterator to all the preferences in 
the group. We're also planning on having subgroups at some point, so a 
group can have one or more subgroups beneath it.

These groups are then registered with the PreferenceManager. When a 
group is registered with the PreferenceManager it will automatically 
show up in the PreferenceWindow in the left TreeView (along with it's 
subgroups as children), and the preferences in that group will appear in 
the right pane when that option is selected. When the window is closed 
all preferences are saved into the model and the listeners are notified 
of the update. Hitting cancel undoes all changes and reverts the 
preferences to their previous state.

Let us know what you think, any feedback is always appreciated :)