Fix problems on destruction

Fix problems on destruction

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.

https://bugzilla.gnome.org/show_bug.cgi?id=611203