SingleMessageWindow: Fix deprecation warning

gajim/data/gui/single_message_window.ui

 <?xml version="1.0" encoding="UTF-8"?>
-<!-- Generated with glade 3.22.1 -->
+<!-- Generated with glade 3.36.0 -->
 <interface>
-  <requires lib="gtk+" version="3.20"/>
+  <requires lib="gtk+" version="3.22"/>
   <object class="GtkImage" id="image1">
     <property name="visible">True</property>
     <property name="can_focus">False</property>
@@ -289,13 +289,13 @@
         <property name="layout_style">end</property>
         <child>
           <object class="GtkButton" id="close_button">
-            <property name="label">gtk-close</property>
+            <property name="label">_Close</property>
             <property name="visible">True</property>
             <property name="can_focus">True</property>
             <property name="can_default">True</property>
             <property name="receives_default">False</property>
             <property name="no_show_all">True</property>
-            <property name="use_stock">True</property>
+            <property name="use_underline">True</property>
             <signal name="clicked" handler="_on_close_clicked" swapped="no"/>
           </object>
           <packing>