wayland: Track text input serials in a way that allows bursting commits

With two separte serials, to track sent requests and confirmations, it's easier to send non-atomic changes as a unit, if forced to. The algorithm should be self-correcting, reducing the risk of leaving the application in an invalid sate.

Closes gtk#1365 (closed) , gnome-shell#585 (closed)

This works because each done sent to the client must cause a commit back. This is implied by the text-input-v3 protocol, but not explicitly stated (I need to write an amendment if this is a good workaround).