[DON'T MERGE] wlr-workspaces implementation

This is my work so far on implementing https://github.com/swaywm/wlr-protocols/pull/35

I decided to first implement the protocol in its current form (with slight modifications as I saw fit), and I'd like feedback on them. If it looks okay I will rename wlr to ext and open a PR against wayland-protocols as well.

The only notable change from the PR is that all synchronization is done at the workspace-manager level (both commit and done are moved there), although we could make them per workspace-group.

About the implementation, it should be very similar to foreign-toplevel-management, in almost all aspects. I am not 100% sure on the commit part (with pending, current & server_state).

If someone wants to test, I have added a wayfire plugin to support this protocol: https://github.com/WayfireWM/wayfire/pull/647