upgraded the manageButton to the new type ViewHead;
started to introduce new terminology:
The view is composed of a "head"and a "body". The body is made of a 
"dashboard" and a ...
changed terminology again:
The view is composed of a handle plus a canvas. 
The canvas is made of a head (or dashboard) plus a body.
introduced abstract class ViewBody and removed related boolean variable
renamed fillPane to fillBody
creating the dashboard in a separate method and cleanedup
moved code for the createButton into separate method
copied the DragAndDrop functionality from the file into the
StructureView
added numViewAdded and numViews() to navigators;
added creationRank to StructureView;
corrected return type of numViews;
updated terminology plus reorganization in JFXNavigator
updated count names: numViewCreated and numViewsOpen
change ViewHandle to be subclass of Pane;
storing that instead of a button in the Navigator's handle container
showing the creationRank in the handle; had to move code around to make
sure that the handle was created AFTER this variable was set