gtkscrolledwindow: Fold kinetic deceleration handling into scroll_event()

gtkscrolledwindow: Fold kinetic deceleration handling into scroll_event()

In order to play along with child widgets that use scroll events for anything
else than scrolling, it will be better to do this in the bubble phase, so
the child widget has an opportunity to GDK_EVENT_STOP the event before we
trigger kinetic scrolling.

This of course won't work for widgets that choose to reimplement scroll event
handling themselves, they should be smart at resorting to GtkScrolledWindow's
scroll event handling.

This fixes kinetic scrolling kicking in too pervasively on widgets that eg.
implement zoom on scroll events.

https://bugzilla.gnome.org/show_bug.cgi?id=753495