demo: change make-demo.py to allow demo dir to have subdirs (for utils).
scene: add SceneItem interface for objects that can be added to an svg.Scene.
event: add an event type for the beginning of a scene.
event: add a event class corresponding to SCENE_BEGIN.
demo/util: add two SceneItem impls (ball, arrow) for upcoming new demo.
scene+demo: replace previous "event" demo with a clickable ball+animation.