Add `focus-window-previous` action

[zeroeightysix]

This action focuses the previously focused window. As a parallel to focus-workspace-previous, this works across outputs, or within the same workspace, and should come in handy for users that have scripts making use of focus-window.

• [x] what should happen when the previously focused window has since been closed?
• [x] todo: test this as daily driver and on a multi-monitor setup

[zeroeightysix]

Been using this for a few days now without issue, so it's ready for a final review :)

[YaLTeR]

Yeah, sorry, I have this checked out, but I've been very busy with uni.

[YaLTeR]

Did some minor cleanups

[YaLTeR]

Hmm, what if we store the previous focused window just in the Niri state itself, rather than on each window? I think that should be simpler and should fix the layer-shell edge cases. I'll try doing it tomorrow when my brain is a bit clearer

[YaLTeR]

Could you please try this branch again? I made the change above.

[zeroeightysix]

Could you please try this branch again? I made the change above.

Will do. This approach also does look nicer.

[zeroeightysix]

Appears to be working as intended!

[YaLTeR]

Thanks!