Use cosmic-workspace-unstable-v1 instead of ext-workspace

Labwc has chosen to vendor and implement Cosmic's workspace protocol. It's very similar to the current ext-workspace draft, except that it's behind on some of the changes that have been made more recently, and still does the following "outdated" things:

• workspace event on the group instead of manager
• workspace_enter and workspace_leave on the group are missing
• wl_array used for capabilities and states rather than a bitfield

I've decided to maintain the existing semantics of our public API that match the semantics of the current ext-workspace draft, as those semantics were so much easier to deal with in applications than the old way of doing things.