Refactor: Create custom class for prefs box order GtkListBoxes

Create a custom class (`PrefsBoxOrderListBox`) for GtkListBoxes, which
are used in the preferences window for holding box order settings.
Having a custom class for these GtkListBoxes is useful, since it can
hold logic related to box order settings. Future commits will bring more
of this logic into this class.

src/prefsModules/PrefsBoxOrderListBox.js

@@ -0,0 +1,42 @@
+/*
+ * This file is part of Top-Bar-Organizer (a Gnome Shell Extension for
+ * organizing your Gnome Shell top bar).
+ * Copyright (C) 2021 Julian Schacher
+ *
+ * Top-Bar-Organizer is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+/* exported PrefsBoxOrderListBox */
+"use strict";
+
+const Gtk = imports.gi.Gtk;
+const GObject = imports.gi.GObject;
+
+const ExtensionUtils = imports.misc.extensionUtils;
+const Me = ExtensionUtils.getCurrentExtension();
+
+var PrefsBoxOrderListBox = GObject.registerClass({
+    GTypeName: "PrefsBoxOrderListBox",
+    Template: Me.dir.get_child("prefs-box-order-list-box.ui").get_uri()
+}, class PrefsBoxOrderListBox extends Gtk.ListBox {
+    /**
+     * @param {Object} params
+     * @param {String} boxOrder - The box order this PrefsBoxOrderListBox is
+     * associated with.
+     */
+    _init(params = {}, boxOrder) {
+        super._init(params);
+
+        this.boxOrder = boxOrder;
+    }
+});