In prep work for exposing some of this to the caller, this adds a new
object that carries the our callbacks including the user data (if any).
This is an internal system only and is only used in the handshake
implementation where we don't have userdata anyway.
The new approach is: the callback has an object with a done() and
destroy() callback and the user data, done() is called when we receive
the message from the protocol, destroy() on destroy regardless whether
we got done() first.
This allows a caller to clean up user data even where the callback was
not triggered because we got disconnected first.
Part-of: <https://gitlab.freedesktop.org/libinput/libei/-/merge_requests/316>
Split the ei_pointer protocol interface into ei_pointer,
ei_pointer_absolute, ei_scroll and ei_button.
This gets rid of the slightly awkward pointer vs pointer absolute
handling. Those were two different capabilities but tied to the same
interface.
Plus it paves the way for devices that are keyboards with scroll
buttons, etc.
The protocol name on an interface is a fixed string that is part of
the ABI since it's used in a few messages (e.g.
ei_handshake.interface_version). To avoid typos, let's expose that
string in the scanner and #define it in the generated sources.
Previously, we'd send one interface_version event for "ei_handshake"
immediately but all others after the client requests handshake.finish.
This was too confusing to document and not clear how it would work, so
let's make this simpler by splitting it up.
There is now a handshake_version event from the server, sent immediately
on connection that denotes the maximum version number for the interface.
And a handshake_version request from the client which must be the first
one by the client.
