Draft: Implement keyboard filtering for input method

[dcz-self]

This is a draft because it's not verified end-to-end and the protocol is still a proposal.

It adds the methods needed for input methods to filter keyboard events before they reach the text input client, as submitted to wayland-protocols

The idea is: the client requests a keyboard and passes it to the input method to change its purpose.

Currently, it's just a bunch of methods the user calls, but I'm not sure this low-level approach is the best. The bind call could create a keyboard automatically for the current seat. Also, the bind/unbind calls could protect the user from protocol errors by remembering if a keyboard is set and returning Result. Would that be an improvement?