Support hints from notification spec 1.2

Even though the specification makes it perfectly clear that you should
query the notification server and only use supported hints, gnome and
libnotify does not.

Given the specification has changed in a non-compatiable way, this
causes image-data/image-path to break.

This patch supports 1.2 fields

Submitted by: Konstantin
BUG: 344885

Reviewed-by: David Edmundson