make preview mode better

[fox]

I'd find it helpful if the preview mode told you how long a sequence was, in minutes and seconds. Maybe that's a bit tricky to do properly these days when markers can change BPM, and when often people include silent notes at the end. I still want to know what I'm getting into when I press play on a sequence though.

this will also come with the redesign, pretty simple and we already have a system for it