Draft: Begin refactoring weston_client into wet_process

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

This MR is mostly mechanical, and begins reworking the weston_client helpers which are largely based around a wl_client, into wet_process which is correctly named and fully encapsulated.

The goal of these changes is to enable the other changes in !932 (closed), where we fully track client state in a predictable way which doesn't involve memory leaks.