Notifications: Fix deprecation warning

gajim/data/gui/popup_notification_window.ui

 <?xml version="1.0" encoding="UTF-8"?>
-<!-- Generated with glade 3.22.2 -->
+<!-- Generated with glade 3.36.0 -->
 <interface>
-  <requires lib="gtk+" version="3.20"/>
+  <requires lib="gtk+" version="3.22"/>
   <object class="GtkEventBox" id="eventbox">
     <property name="name">NotificationPopup</property>
     <property name="visible">True</property>
@@ -74,7 +74,7 @@
                       <object class="GtkImage">
                         <property name="visible">True</property>
                         <property name="can_focus">False</property>
-                        <property name="stock">gtk-close</property>
+                        <property name="icon_name">window-close</property>
                         <property name="icon_size">1</property>
                       </object>
                     </child>