Extend v2 interface to support selection updates

This is the Sushi side of what's needed to fix #5 (closed). We expose a new signal and a new property over the new v2 D-Bus interface, which Nautilus will read/connect to in order to update the preview window when selection changes, and change selection when directional arrows are pressed in the Sushi window.

Since this depends on the v2 interface, it's stacked on top of !7 (merged).