Switch focus on URL open

[Saviq]

This is probably a contentious topic while the compositor doesn't know about an app's request to open another (e.g. calling xdg-open), as it's working against #2586. But today you click a link in one app, it opens in another, and you have no idea.

I know we had designs for this in mirclient, and these likely need to migrate into the new world.

[RAOF]

This is actually a request to implement the xdg-activation protocol that we contributed input to.

[RAOF]

(Plus waiting for clients to actually use that protocol)