libei integrations in the XDG RemoteDesktop and InputCapture portals

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…