StLabel StTooltip: we need to unparent the child, not destroy
  it since Clutter is buggy when destroying actors not in
  a ClutterContainer and leaks them.

StScrollBar: Be robust against being disposed multiple times,
  which can happen, and in fact, normally happens when destroying
  the parent.

StScrollView: Implement remove() for the hscroll and vscroll members,
  and just destroy them in dispose() and let them be removed.
  unparent the shadows, instead of just unref'ing them directly.