Make Sushi preview change and follow Nautilus selection
This is the Nautilus side of what's needed to fix sushi#5 (closed). We use a new signal and property that Sushi exposes in its D-Bus v2 interface to update the preview window when selection changes, and to change the Nautilus selection when arrow key events are received in Sushi. This should be merged together with the corresponding Sushi MR (sushi!8 (merged)), even though code is implemented in a way that it will gracefully fall back in case an older Sushi is found.
It however depends on !469 (merged) since that adds support for the v2 NautilusPreviewer interface.