window: Add accessible-name for the Search button

The search toggle button in the upper-left corner of the Window
lacks an accessible name. Thus when it gains focus, screen readers
cannot present what this widget does; only that it is a toggle button.

https://bugzilla.gnome.org/show_bug.cgi?id=790625

shell/window.ui

@@ -134,6 +134,11 @@
                 <property name="can_focus">True</property>
                 <property name="receives_default">True</property>
                 <property name="no_show_all">True</property>
+                <child internal-child="accessible">
+                  <object class="AtkObject">
+                    <property name="accessible-name" translatable="yes">Search</property>
+                  </object>
+                </child>
                 <child>
                   <object class="GtkImage">
                     <property name="visible">True</property>