[441202] Construct hover handles based on hover handle geometry.
[438734] Implement tooltip for nodes.
[441202] Refactor FXDefaultHandlePartFactory to support hover handles.
[441202] Set viewer contents in the JavaFX application thread to prevent CME.
[441202] Fix handle hover for Zest.FX.

Hovering a "hover-handle" changes the hover-model, and therefore removes the
"hover-handle". That's why hovering a "hover-handle" cannot be done via the
hover-model, at the moment.
[441202] Implement prune/expand via hover-handles.