foreign-toplevel-management: add urgency hint

The motivation comes from labwc's issue.

In KDE, when a user opens a URL with xdg-open and there's already an existing browser window, the focused window remains unchanged. Instead, the panel item for the browser turns orange, indicating it needs attention. Allowing wlroots panels (e.g. Waybar and sfwbar) to implement similar behavior would be beneficial.

Sway shows urgency hints by changing titlebar color to red, which might be sufficient for it. However, for many Wayland compositors that allow window minimization, the panel item is the best place to show urgency hints in my opinion.

Possible solutions include:

1. Adding needs_attention to the state enum.
2. Adding needs_attention event to the zwlr_foreign_toplevel_handle_v1 interface.

While (1.) looks natural and will work seamlessly with XWayland windows, (2.) allows panel clients to decide when to drop the urgent state (typically when the urgent window is activated), slightly simplifying the compositor's task. In addition, I think needs_attention doesn't need corresponding set_needs_attention request, so it's a bit different from existing minimized/maximized/activated/fullscreen states.

If we choose (1.), it will also be worth considering adding always_on_top/always_on_bottom/sticky/shaded states too (suggested here).

(urgent hint in KDE panel)