gtk/scrolledwindow: Handle full scroll sequences only

gtk/scrolledwindow: Handle full scroll sequences only

In order to do this, leverage smooth scroll handling into the capture
phase scroll controller, controlled by ::scroll-begin in the propagation
phase controller.

There's 2 cases here:
- A child widget handles scroll. The scrolled window does not get
  ::scroll-begin, the child widget handles the full scroll sequence.
- No child handles scroll, the scrolled window gets ::scroll-begin,
  and transfers control of scrolling to the capture phase controller.
  As scrolling is performed, the pointer may fall into scrollable children,
  but the scrolled window will be capturing the scroll events, so these
  won't be seen by the child widgets.

Fixes: https://gitlab.gnome.org/GNOME/gtk/-/issues/593