remote-desktop: add the ability to communicate via an EIS socket

[whot]

This is intended as replacement for the NotifyFoo methods. libei
provides a more flexible and powerful method of sending input events to
the compositor.

A new method ConnectToEIS requests a file descriptor from the compositor
which can then be plugged into libei.

Once established, the communication between compositor and application
is direct, without the need to go through the portal process(es).

To avoid ambiguities between NotifyFoo and input events sent via libei,
any application that uses an EIS connection cannot use the NotifyFoo
methods.

One notable comment: I decided to pass the EIS connection down through the impl.portal instead of pipewire's approach of having the portal connect directly. Unlike the single pipewire daemon, an EIS implementation can be provided by multiple processes, so I figured it's easier to pass this down as close as possible to the entity that knows who the EIS implementation

Eventually (and once libreis is ready) this will have the hooks to auto-set the app-id on the EIS connection as well though strictly speaking it's not necessary since the app-id is passed down to whoever needs to set up the socket anyway.

Noteworthy: there is no keysym event in libei at this point, so there's no equivalent to NotifyKeyboardKeysym

cc @jadahl

[whot]

ftr, we have a working (though not yet complete) use-case in this input-leap PR

[whot]

Marking this as ready. There's no immediate libei dependency other than the socket being passed, so even if libei takes another few weeks for the official "it's stable" API this bit is ready.

Squashed the commits together, there is no purpose keeping the various bugfixes we had in the git history after the merge.

[whot]

ftr, libei is now declared stable. Not that it matters for this PR since we don't actually do anything with libei directly.

[jadahl]

All lgtm, I think this is ready to land. Can consider tweaking the error message, but nut super urgent.