[FEATURE] Support xdg-activation

[bbb651]

Description of the requested feature

When eww rans external commands, they aren't given a valid XDG_ACTIVATION_TOKEN (usually they will get an invalid inherited from eww because it doesn't explicitly clear it), potentially leading to focus not switching if a new window is created, e.g. with niri's strict-new-window-focus-policy.

I think the way this to do in gtk is using AppInfo.create_from_commandline then AppInfo.launch which will handle activation automatically, I'm not sure what implications this on launch speed and behavior, alternatively we can try to access xdg_activation_v1 ourselves with WaylandDisplay.

Proposed configuration syntax

None, should be applied automatically to all commands that are executed.

Additional context

No response