wayland: tasklist: Fix monitor detection when panel is remapped

gdk_display_get_monitor_at_window() is unreliable on wayland, because when the panel is remapped it usually first appears on one monitor before being moved to another, and there is no signal or event to indicate when this process has finished. So it's best to use gtk_layer_get_monitor() whenever possible, as we always use gtk_layer_set_monitor() upstream.