Workspaces: Add a 'populated' class

[ItsLemmy]

I would love to be able to distinguish populated vs non-populated workspaces so we can theme empty workspaces differently via CSS.
I'm using Niri with named workspaces and I miss this feature that I used to have with AGS.
It's way more efficient for me to be able to tell which workspace have windows on them.
Thank you for considering this feature request!

[JakeStanger]

Yes that makes sense in the context of Niri. I've logged as #1078