[gnome-calculator] math-window: Add the Undo button in the header bar

[Robert Roth <robertroth src gnome org>]

commit 0f73fa2a70cad19b4280849f417004127ffb501a
Author: Adrien Plazas <kekun plazas laposte net>
Date:   Fri Jan 10 15:28:30 2020 +0100

    math-window: Add the Undo button in the header bar
    
    It will be removed from the advanced buttons as it is refactored in the
    next commit.

 src/ui/math-window.ui | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)
---
diff --git a/src/ui/math-window.ui b/src/ui/math-window.ui
index 4ec1ac9d..96dc68bf 100644
--- a/src/ui/math-window.ui
+++ b/src/ui/math-window.ui
@@ -167,6 +167,22 @@
                 </style>
               </object>
             </child>
+            <child>
+              <object class="GtkButton" id="undo_button">
+                <property name="visible">True</property>
+                <property name="can_focus">True</property>
+                <property name="receives_default">True</property>
+                <property name="valign">center</property>
+                <property name="use_underline">True</property>
+                <property name="focus_on_click">False</property>
+                <property name="label" translatable="yes">Undo</property>
+                <property name="tooltip_text" translatable="yes">Undo [Ctrl+Z]</property>
+                <property name="action_name">win.undo</property>
+                <style>
+                  <class name="text-button"/>
+                </style>
+              </object>
+            </child>
             <child>
               <object class="GtkMenuButton" id="primary_menu_button">
                 <property name="visible">True</property>