This is another attempt at adding support for libei in mutter.
Initially, I filed !1436 (closed) two years ago but since then the libei API has changed a lot, the logic as well, meaning that !1436 (closed) would require more than just a rebase.
In the meantime, Peter (@whot) also gave it a try in his own branch https://gitlab.gnome.org/whot/mutter/-/tree/wip/ei (but didn't file a MR afaik).
While Peter's branch is also almost a year old (and based on mutter from GNOME 40), it's still closer to the current libei API than !1436 (closed) was, so I figured it might be easier to just start from @whot's branch, rebase it against the current mutter code and update it to match the current libei API.
This also includes @jadahl's branch for InputCapture support and adds some more fixes/updates from me on top for good measure.