Multi EIS in remote desktop and PW stream integration

This should make libei usable with all screencast types, including "area" and "window". It also makes sure the device selection done in the portal is respected. On main right now libei always mirrored logical monitors as absolute devices, but this is now only done for input-only remote desktop sessions, i.e. ones without any PipeWire streams attached to them.

This is related to https://github.com/flatpak/xdg-desktop-portal/pull/1080/, xdg-desktop-portal-gnome!101 (merged), xdg-desktop-portal-gnome!102 (merged), and https://gitlab.freedesktop.org/libinput/libei/-/merge_requests/249.

While GNOME GitLab was down, it saw some review over here: https://github.com/jadahl/mutter/pull/2