Allow syncing individual files in Differences screen

With [this](https://github.com/ASIDataScience/sml-sync/pull/5) PR, we now have an itemized list of actions to take when syncing up or down. We could extend that to allow marking individual files, and then have an action to just sync those files, rather than the entire tree.

This would make the entire process feel much safer: you know exactly what you want to sync.