Tie color of update notification emblem to highest urgency update

### Problem

AppCenter's emblem is always red when there are updates available. While a good default (updates can be very important), it would be nice to have more information as to when something _can_ be updated versus _needs_ to be updated.

### Proposal

Change the emblem's color to reflect the highest available update's urgency

### Prior Art (Optional)

OS system updates in Settings (will add screenshot when I can get one)