Prefer the term "window" over "client" in Workspace

The main bulk of this MR was created using clang-rename and a text editor.

Workspace::clientList() and Workspace::allClientList() were left as is intentionally. Renaming Workspace::allClientList() to Workspace::allWindowList() would create more confusion than what we have now. Maybe unify all client lists in a single one?

It partially touches autotests. Perhaps client -> window can be performed using some fancy script that uses clang-rename. Not very critical, can be addressed later.