dbusServices/extensions: Remove event box

In GTK4, all widgets are reactive, and therefore GtkEventBox has
been removed. In order to make the upcoming GTK4 port a bit cleaner,
remove the expander's event box now.

Part-of: <!1495>

js/dbusServices/extensions/css/application.css

-.expanded { border: 0 solid @borders; border-bottom-width: 1px; }
+.expander { padding: 12px; }
+.expander.expanded { border: 0 solid @borders; border-bottom-width: 1px; }
 .expander-toolbar {
   border: 0 solid @borders;
   border-top-width: 1px;

js/dbusServices/extensions/ui/extension-prefs-dialog.ui

@@ -62,28 +62,22 @@
                             <property name="hexpand">True</property>
                             <property name="orientation">vertical</property>
                             <child>
-                              <object class="GtkEventBox" id="expander">
+                              <object class="GtkBox" id="expander">
                                 <property name="visible">True</property>
+                                <property name="spacing">6</property>
+                                <style>
+                                  <class name="expander"/>
+                                </style>
                                 <child>
-                                  <object class="GtkBox">
+                                  <object class="GtkImage" id="expanderArrow">
                                     <property name="visible">True</property>
-                                    <property name="margin-start">12</property>
-                                    <property name="margin-end">12</property>
-                                    <property name="margin-top">12</property>
-                                    <property name="margin-bottom">12</property>
-                                    <property name="spacing">6</property>
-                                    <child>
-                                      <object class="GtkImage" id="expanderArrow">
-                                        <property name="visible">True</property>
-                                        <property name="icon-name">pan-end-symbolic</property>
-                                      </object>
-                                    </child>
-                                    <child>
-                                      <object class="GtkLabel">
-                                        <property name="visible">True</property>
-                                        <property name="label" translatable="yes">Technical Details</property>
-                                      </object>
-                                    </child>
+                                    <property name="icon-name">pan-end-symbolic</property>
+                                  </object>
+                                </child>
+                                <child>
+                                  <object class="GtkLabel">
+                                    <property name="visible">True</property>
+                                    <property name="label" translatable="yes">Technical Details</property>
                                   </object>
                                 </child>
                               </object>