Override app id in nested-wayland context

[andrewbaxter]

• Description:
  Right now when sway is started as a nested compositor its windows get the id "wlroots". I'd like to be able to define styling rules, etc based on the app id and need a unique app id in order to do this. Would it be possible to allow overriding this, as a command line option, config file option, or environment variable?

I think this would need a wlroots change around here: https://gitlab.freedesktop.org/wlroots/wlroots/-/blob/master/backend/wayland/output.c#L867

	wlr_wl_output_set_title(&output->wlr_output, NULL);

	xdg_toplevel_set_app_id(output->xdg_toplevel, "wlroots");
	xdg_surface_add_listener(output->xdg_surface,
			&xdg_surface_listener, output);

FWIW I opened the issue here because this is where the ultimate use-case is.