improved the structure of the navigation code: creating a private method
to create the corresponding buttons in the rod and view button panels
(in the top area of the window)
created DomainView class to manage a domain view within the Navigator
removed constructors from DomainView with low level arguments useful
only for constructing the contained Domain;
new method activate in DomainView;
removed static modifier from fields in
JFXNavigator.
highlighted the SplitPane.setDividerPositions and cleaned up;
removed another static modifier (why were they there?)
replaced the code in the navigator with the activate call in DomainView
created viewExists boolean variable to prevent multiple views being
called; renamed buttons;
temp
put some comments in ArrayListDomainToken. 
It is supposed to be a Form (that's why it extends a Java util class
rather than AbstractStructure). 
Must ensure that all its elements are also Forms (see remarks therein)
introduced a Singleton abstract class that implements CollectionDomain;
to be fixed, probably
created a Singletom class to represent a CollectionDomain with on
element