Add a common interface to our animated panes.

Even though the new panes extend different javafx layouts,
they have some common fields (simulation, strength, etc.)
and should provide these via a common interface.
Make the VBox example toggle between V/HBox.

This merely requires creating a new pane, reinitializing
as needed, and adding the nodes to the new pane.

It reveals a significant cleanup flaw:
The simulator does not simply drop nodes when they are removed
from the managed layout. When we move nodes between different
animated panes, both simulators will fight over these nodes.

Currently using a workaround by permanently disabling the simulator
when the old pane is replaced, but ideally PhysLayout should watch
over its nodes and drop them when their parent changes.