make plug mode default instead of windowed

I propose to use:
...
        Gtk.init(ref args);
        if (args.length > 1 && args[1] == "window") {
            new MainWindow();
        } else {
            var plug = new Plug ();
            plug.register(_("Startup Applications"));
            plug.show();
        }
        Gtk.main();
...

instead of the current approach since when using this plug without your run_* scripts it opens windowed when opened from switchboard. Making this change allows you still to use windowed for development/testing while it properly integrates into switchboard otherwise.