Currently, the scroll handler obeys to the page increment, but instead
of using the step increment for more precise scroll, it uses an
hardcoded one tenth of the page increment.
Use the step increment instead since it has been filled with sensible
values by ArdourFader's users.
The queued resize will only happen trigger a size-request when the
widget is realized, and on_size_request() calls ensure_layout().
Moreover, this over protection meant that sometimes a resize wasn't
noticed by the button containers.
