refactor(bar): use Gtk enums for position and orientation

Ensure that the position and the corresponding CSS class on window are
always set.

src/modules/river/tags.cpp

@@ -87,7 +87,7 @@ Tags::Tags(const std::string &id, const waybar::Bar &bar, const Json::Value &con
       control_{nullptr},
       seat_{nullptr},
       bar_(bar),
-      box_{bar.vertical ? Gtk::ORIENTATION_VERTICAL : Gtk::ORIENTATION_HORIZONTAL, 0},
+      box_{bar.orientation, 0},
       output_status_{nullptr} {
   struct wl_display *display = Client::inst()->wl_display;
   struct wl_registry *registry = wl_display_get_registry(display);