ScaleManager#onFullScreenInit

This signal is dispatched when fullscreen mode is ready to be initialized but
before the fullscreen request.

The signal is passed two arguments: scale (the ScaleManager), and an object in the form {targetElement: DOMElement}.

The targetElement is the fullScreenTarget element,
if such is assigned, or a new element created by createFullScreenTarget.

Custom CSS styling or resets can be applied to targetElement as required.

If targetElement is not the same element as fullScreenTarget:

• After initialization the Display canvas is moved onto the targetElement for
  the duration of the fullscreen mode, and restored to it's original DOM location when fullscreen is exited.
• The targetElement is moved/re-parented within the DOM and may have its CSS styles updated.

The behavior of a pre-assigned target element is covered in fullScreenTarget.