StScrollable: Document how size negotation now works between the
parent and scrollable child.
StBoxLayout: Adapt to the new contract for how size negotiation
works; in particular, handle being allocated less than the
minimum size when scrolled and treat the minimum size as the
size of the scrolled area in instead of the natural size.
StScrollView: Substantially rewrite with fixes including:
- Implement new size negotation contract; this allows us
to determine scrollbar visibility without having to
connect to the adjustment.
- Implement all ALWAYS along with the existing NEVER/AUTO
- When hiding and showing scrollbars and shadows, don't
hide and show widgets, just turn on and off including them
in pick and paint. This avoids queueing relayouts.
- Cleanups for the code for connecting to adjustments,
for changing policy, and for turning on and off shadows.
scroll-view-sizing.js: New test case for StScrollView, allowing
resizing the scroll view interactively, changing the scrollbar
policies and turning shadows on and off.
https://bugzilla.gnome.org/show_bug.cgi?id=611740
