GtkScrolledWindow: Allow scrolling without bars

Add a new policy, GTK_POLICY_EXTERNAL, which hides the scrollbar,
but does not force the scrolled windows size to be determined by
its child. This can be used to keep two scrolled windows in sync,
while sharing a single scrollbar.

https://bugzilla.gnome.org/show_bug.cgi?id=730730
Add an example for synchronized scrolling

This example uses GTK_POLICY_EXTERNAL to share a scrollbar among
multiple scrolled windows.
GtkScrolledWindow: Introduce a helper function

Instead of making assumptions about enum value ordering,
introduce a helper function to determine policy values
for which the scrollbar may be visible.