Port to libhandy master
The libhandy part is merged, this should be ready.
WIP because:
- It needs libhandy!456 (merged) which is not merged yet
- Even when it's merged it won't be in a release for some time, so we'd need master
- Since we need master, it needs to move to GNOME infra first to be added to g-b-m
But while I'm pinging @aplazas about migrating, would be better to have a MR opened rather than have it gather dust locally.
Edited by Alice Mikhaylenko