Change how Ardour works with region crossfades.

• Destructive crossfade length is used when an operation on a region is destructive, such as when recording in a track is in tape mode.
• When Region fades active is checked, the region fades set up in the mixer are used during playback. When unchecked, the fades are ignored.
• When Region fades visible is checked the region fades are visible in the the Editor.