Feature Request: Add a single track by default at launch

Hey there, I am looking into using Mini-Cut for some personal video editing work.
A small issue that has got me the last few times that I have started this program is the fact that one needs to manually add a track in order to do anything. It feels like the first track should be added by default since you need at least one track to do anything. I poked through the code base and I found that adding the following line to
`MiniCutState.swift` initialized the interface with a single track:

`init() {
self.timeline.tracks.append(Track(name: "Track \(self.timeline.tracks.count + 1)"))
}`

Please let me know if there is interest in adding such a feature let and I can submit a pull request.