Instead of using a custom apply_transform(), paint(), and pick()
implementations, we can simply apply a transformation to the children of
a ScrollActor.
https://bugzilla.gnome.org/show_bug.cgi?id=686225
When the transition was removed from the scroll-actor manually,
to cancel a not-finished animation, the transition struct member
wasn't reset to NULL.
This fixes this problem, and removes the need for the struct member
to be reset manually when animation has completed.
https://bugzilla.gnome.org/show_bug.cgi?id=676334
Chaining up to the parent's implementation of pick() is not enough: we
need to paint our children explicitly because of the compatibility mode
checks we use to avoid breaking custom containers.
https://bugzilla.gnome.org/show_bug.cgi?id=676088
We need to clip the children during picking as well as we do when
painting, to avoid reactive children outside of the visible area
receiving events.
This also allows us to refactor some common code into proper functions.
