fix region fade/env XML save - fixes #5353

fix region fade/env XML save - fixes #5353

In rev 12740 FadeIn/Out became stateful properties which are automatically
saved when Region:state() calls Stateful::add_properties().

AudioRegion::state() called Region:state() AND Stateful::add_properties()
which added a 2nd redundant copy to the XML.

Finally AudioRegion::state() adds custom serialization for Fades.
Furthermore this custom serialization also used names which did not
match the Property name.

git-svn-id: svn://localhost/ardour2/branches/3.0@14117 d708f5d6-7413-0410-9779-e7cbd77b26cf