Adds support for cosmic-workspace-v2 pin, unpin, move_after, and
move_before requests.
Both features need some work with workspaces span displays mode, so that
will need more fixes later.
We also want to generate a unique id for pinned workspaces to send in
the ext-workspace-v1 protocol. But that isn't a strict requirement for
anything. So I haven't yet fully implemented that. We'll also want to
persist other things, like workspace naming when that's added.
Overall, though, with separate workspaces per display, this is working
pretty well.
