Ei protocol support using `reis`

[ids1024]

The Ei protocol is needed for the xdg-desktop-portal RemoteDesktop portal to emulate input devices, as well as by the InputCapture portal for Synergy-like uses (input-leap supports Wayland with this portal).

This is quite incomplete, but the type-text example in reis now works in Anvil. Reis could still use somewhat better higher-level servere-side APIs.

It might make sense if ei exposed seats and devices matching those in Smithay (or would compositors not always want to do that?). Not sure how best to handle that.

Receiver contexts for the InputCapture portal are also a bit more complicated to implement. Those involve capturing input once the cursor crosses outside the display. That isn't implemented at all here yet.