Draft: wet_process: Consistently use encapsulated wet_process

[this MR is a subset of !932 (closed), and is currently on hold]

This series continues using wet_process more consistently, rather than open-coding different parts of it in each user.

At the end of this series, all wet_process users allocate a wet_process on the heap, and use this to encapsulate the wl_client created for that process. wet_process gains its own destroy handler with userdata, rather than requiring users to install a destroy listener on the wl_client it creates. This is now used by desktop-shell, the input method, the screenshooter, and Xwayland.

Xwayland still has its own process-forking infrastructure to support signal-safe forks, as does the desktop-shell client. These will be merged in the next MR, now that we have all the users other than Xwayland consistently using wet_process creation.