Add support for fullscreen-shell

[emersion]

This allows:

• wlroots to be used nested in another compositor
• wlroots to be used to have a single fullscreen client

There are many use-cases for this, including: a remote screen session, things similar to Steam big picture sessions, a lockscreen with a dynamic background…

---

wlroots has migrated to gitlab.freedesktop.org. This issue has been moved to:

https://gitlab.freedesktop.org/wlroots/wlroots/-/issues/1370