proto: add a version argument to ei_connection.sync

This is the only request that creates a new object but doesn't specify the version for that object, courtesy of copy/paste from the wayland protocol. In libei/libeis this a bit was hidden away so it didn't get noticed - but it was already buggy: libei would always hardcode to version 1 but libeis would take whichever ei_callback version was agreed upon during handshake. This version could be higher than 1.

This is a protocol break but we're still pre-1.0, there are very few people that will be affected by this and it's better than having to carry this bug around for years.

Fixes #35 (closed)

cc @ids1024 (you're the only one currently working on bindings, afaict)

cc @jadahl, @ofourdan (who worked on the compositor/portal bits)