Picture Rating

It would be nice to set the rating via keyboard 0-5.
Also to extend the picture menu with rating (to see the all possibilities).

The only problem is that pure number shortcuts are reserved for accessing collections that have no explicit shortcut, so we’d have to use something different (probably using certain modifier keys in addition to the digit).
I’ve added in internal issue for this now: #423

Preview 4 has added menu entries with default shortcuts for setting ratings (Cmd+0 to Cmd+5 on macOS and Ctrl+0 to Ctrl+5 on other systems).