WIP feat(interactive): Add interactive diff

[aawsome]

Adds the possibility to compare two snapshots interactively. You have to mark exactly 2 snapshots and press D to get to the diff view.

Also allows to recusively compute added/removed space for dirs and subdirs using s (like in #1479)

Open:

• [x] window label (snapshots; current dir(s))
• [ ] show details with i (entry) / I (snapshots)
• [x] maybe rework diff command to use NodeDiff?
• [ ] When recursing over sub-dirs, also determine if dirs are equal w.r.t to only metadata-changes.