Add the transient seat protocol

This is a new protocol for creating short-lived seats for remote users, by allowing clients to create seats which will be automatically removed as soon as the client disconnects. If a client was fully responsible for removing its own seats, some unused seats might be left behind if the client crashes or is killed.

TODO:

• Review: @emersion
• ACKs: @emersion
• 2 implementations:
  • sway & wlroots: https://github.com/swaywm/sway/pull/5921, wlroots/wlroots!3870
  • wayvnc: https://github.com/any1/wayvnc/tree/transient-seats