Add icons to close and help buttons in prefs

The icons were lost during the transition from GtkStock.

data/interfaces/xfpm-settings.ui

 <?xml version="1.0" encoding="UTF-8"?>
-<!-- Generated with glade 3.18.3 -->
+<!-- Generated with glade 3.22.1 -->
 <interface>
-  <requires lib="libxfce4ui" version="4.5"/>
-  <requires lib="gtk+" version="3.10"/>
+  <requires lib="gtk+" version="3.20"/>
+  <requires lib="libxfce4ui-2" version="4.12"/>
   <object class="GtkAdjustment" id="brightness-inactivity-on-ac-adjustment">
     <property name="lower">9</property>
     <property name="upper">120</property>
@@ -74,6 +74,16 @@
     <property name="step_increment">1</property>
     <property name="page_increment">5</property>
   </object>
+  <object class="GtkImage" id="image1">
+    <property name="visible">True</property>
+    <property name="can_focus">False</property>
+    <property name="icon_name">help-browser</property>
+  </object>
+  <object class="GtkImage" id="image2">
+    <property name="visible">True</property>
+    <property name="can_focus">False</property>
+    <property name="icon_name">window-close</property>
+  </object>
   <object class="GtkAdjustment" id="light-locker-late-locking-adjustment">
     <property name="lower">1</property>
     <property name="upper">120</property>
@@ -251,6 +261,7 @@
                 <property name="visible">True</property>
                 <property name="can_focus">True</property>
                 <property name="receives_default">True</property>
+                <property name="image">image1</property>
                 <property name="use_underline">True</property>
               </object>
               <packing>
@@ -265,6 +276,7 @@
                 <property name="visible">True</property>
                 <property name="can_focus">True</property>
                 <property name="receives_default">True</property>
+                <property name="image">image2</property>
                 <property name="use_underline">True</property>
               </object>
               <packing>