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/prefs-box-order-list-box.ui

@@ -0,0 +1,25 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!--
+  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/>.
+-->
+<interface>
+    <template class="PrefsBoxOrderListBox" parent="GtkListBox">
+        <property name="selection-mode">none</property>
+        <property name="show-separators">True</property>
+    </template>
+</interface>