st-scroll-bar: Set the handle as a child of the bar, not the trough

The handle was a child of the trough, but it was allocated and painted
like it was a child of the bar. This will wreak havoc when we port over
to the new Clutter API, so let's just make it a child of the bar.

https://bugzilla.gnome.org/show_bug.cgi?id=670034