Expose JFXRepresentation's ToolkitListener in DIsplayRuntimeInstance

[daykin]

Two additions that allow for plugins to track the runtime state.

1. Add a FutureTask called representation_init so others who hear about the RuntimeInstance from e.g. a ListChangeListener can wait until the display is loaded with something interesting before dealing with it. An interested client would call DisplayRuntimeInstance.getRepresentation_init().get() to block until the instance is ready.
2. Add methods to add and remove ToolkitListeners to the instance's underlying JFXRepresentation.

For consideration: The side-effect of (2) is that anyone with a valid reference to a (perhaps critical) toolkit listener can mess with it from an additional place besides JFXRepresentation. However, JFXRepresentation.addListener/removeListener are already public, so this isn't exactly bypassing any built-in separation of concerns.

[kasemir]

A slightly more canonic phrasing for a future or future task that returns nothing might be
FutureTask<Void> instead of FutureTask<Object>. With Object, you wonder what it is.

[daykin]

A slightly more canonic phrasing for a future or future task that returns nothing might be FutureTask<Void> instead of FutureTask<Object>. With Object, you wonder what it is.

Changed to FutureTask, undid whitespace changes and moved unblocking the FutureTask to just after the call to awaitRepresentation()