libei integrations in the XDG RemoteDesktop and InputCapture portals

Summary

Turns out it's been years since I've talked about eggs, so let's change this. libei is, of course, the library for Emulated Input[1]. This post is mostly a refresher because it's been so long and a short summary of some of the work we've done so far, in preparation for some more posts that come soon. libei is a transport layer for logical input events, unlike libinput which is a hardware abstraction layer. In libinput's case the device's firmare/kernel pass events that are somewhere on the sanity spectrum, libinput tries to make sense of those and then we convert those to logical events to be consumed by the next layer (typically the Wayland compositor or Xorg). This is how e.g. "touch down at position x1/y1, touch up at position x1/y2" is converted into a button click event if touchpad tapping is enabled. Or maybe into nothing if we find it was an accidental palm touch. libei works purely on the logical level - you as the libei client pass logical events to the EIS (Emulated Input Server) implementation (typically the compositor). No guesswork, you say button click, EIS gets a button click. libei supports a "sender" and "receiver" mode, depending on whether events are sent to the EIS implementation (input emulation) or receive from the EIS implementation (input capture). libei is designed for the Wayland stack but there are zero requirements for Wayland on either the client or the EIS implementation. Core to libei's design is that the EIS implementation is in control of virtually everything, it decides which devices are available to the client, when those devices can send events, etc. Much like the compositor is in charge when it comes to physical devices - if a compositor decides a physical device doesn't exist, a Wayland client cannot get events from it. Since the original proposal (again, [1]!) we've been busy bees and libei is now a part of the XDG Remote Desktop portal and the XDG Input Capture (both since version 1.17, mid 2023). In both cases the portal is f...