Add support the ext_workspace_manager_v1 protocol

[mark-herbert42]

xfce4-panel complains about
** Message: 18:17:38.346: Your compositor does not support the ext_workspace_manager_v1 protocol
Wayfire - so here is one more candidate to implement, and to improve xfce4 experience. Looks like xfce4 switched to labwc and have plans for own compositor - but I dont think those compositors will be even near to wayfire in features...

[ammen99]

For reference, #647 (comment) is a WIP implementation, pending a wlroots request (and we will probably need a wlroots release before we can add it upstream).