Make WheelPane implement PhysicalPane.
Handle WheelPanes without center nodes.

If there is no center, then the nodes should be
tethered to a fixed point in the origin of the pane instead.
Mouse control in Panes!

This requires modifying the JFXtras MouseControlUtil slightly:

 - since layout managers control their managed children's
   layoutXY, and we use translateXY to move nodes around on the screen,
   mouse movement must also be done in translateXY.

 - toFront() has too many side effects on managed nodes, like
   reordering in VBoxes and HBoxes; therefore nodes are no longer moved
   to the front when being dragged.

WheelPaneExample and VBoxExample have been updated to allow mouse movement.