st: Make st_scroll_view_update_fade_effect() public

Using fixed fade offsets is not always appropriate, this will allow
to set them from code instead.

https://bugzilla.gnome.org/show_bug.cgi?id=707409
st: Add StScrollViewFade:fade-edges

Add a new property which controls whether edge areas are excluded
from the effect (the default and current behavior), or not.

https://bugzilla.gnome.org/show_bug.cgi?id=707409
appDisplay: Use a ScrollView for pages

While we obviously don't want any scrollbars, its fade effect
will give us some extra polish when switching pages.

https://bugzilla.gnome.org/show_bug.cgi?id=707409
appDisplay: Fix return value for _onScroll

ClutterActor::scroll-event has a boolean return value to indicate
whether the event has been handled, or event emission should continue.
Now that we are using an StScrollView, we depend on this to avoid
propagating the event to the view's own handler.

https://bugzilla.gnome.org/show_bug.cgi?id=707409