Support `categories` for notifications
The fdo notifications spec supports adding a category to notifications for notifications. It would be great to have that supported in gio since this helps notification deamons to select specific feedback (e.g. on mobile platforms whether to use LED or haptic feedback based on the users current feedback setting and whether the session is currently active or not).
This is somewhat orthogonal to #1340 since the notification daemon should have lots of leeway to select the right types of feedback rather than being very specific (also it's not restircted to sounds).
I can work on that but would like to check if this sounds reasonable at all.