scroll-view: Support GTK_POLICY_EXTERNAL

GTK+ added a new PolicyType which currently triggers compiler warnings
about unhandled values in switch statements. We also have a use case for
it already, so add support for the new policy type.

https://bugzilla.gnome.org/show_bug.cgi?id=739379
appDisplay: Use EXTERNAL scroll policy

Now that we support the new policy type, we can just use it instead
of hiding the scrollbar.

https://bugzilla.gnome.org/show_bug.cgi?id=739379
Bump version to 3.15.1

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