Add ext-workspace

This is an updated version of the wlr-workspace-unstable-v1 protocol. Original work here: https://github.com/swaywm/wlr-protocols/pull/35

The intention of the protocol is to expose information about workspaces in the compositor and associate them with outputs:

• A compositor exposes one or more workspace groups
• Each workspace group is comprised of several workspaces
• Each workspace group is "visible" on a few outputs

For example, in Wayfire, there is one workspace group per output, because the workspaces on each output are independent.

In GNOME, one would probably also have a workspace group per output, and all of them except the primary output's workspace group will have just one workspace.

Clients would be able to use this protocol to implement workspace switching widgets in panels, command line tools used to focus a workspace and similar.

ACKs:

• COSMIC

• Mir

• Reviewed

Implementations:

• Waybar
• wlroots
• libxfce4windowing
• labwc
• cosmic-comp (cosmic-vendored)
• labwc (cosmic-vendored)
• sfwbar (cosmic-vendored)