when one region is moved and the display is overlaid; use a std::map for an AudioStreamView's crossfades
list so that add_crossfade can find whether a CrossfadeView already exists without looking through
the whole list.
git-svn-id: svn://localhost/ardour2/branches/3.0@5571 d708f5d6-7413-0410-9779-e7cbd77b26cf