Fairly major change to the way in which crossfades are handled;

Fairly major change to the way in which crossfades are handled;
they are now done with region fades, rather than separate objects.
After this commit, Ardour will try to convert your session files
to the new crossfade format, but will make a backup in your
session folder first.

If you have works in progress using Ardour 3 it is
***STRONGLY RECOMMENDED*** that you back up session files before
updating to this commit.

git-svn-id: svn://localhost/ardour2/branches/3.0@11986 d708f5d6-7413-0410-9779-e7cbd77b26cf