GTK3: Adding accessible-label to an image button

Hello,
Now I am trying to understand how to add an accessible label to a button 
with an icon instead of a label. My aim is to make the button label 
invisible, show the icon and set only the accessible-label property.

First I have tryed adding an accessible child to the button as follows 
however this is still in accessible to orca:


               <object class="GtkToolButton" id="play-button">
                 <property name="use_action_appearance">False</property>
                 <property name="related_action">play-action</property>
                 <property name="visible">True</property>
                 <property name="can_focus">False</property>
                 <property name="icon_widget">play-image</property>
                 <child internal-child="accessible">
                   <object class="AtkObject" id="play-button-atkobject">
                     <property name="AtkObject::accessible-name" 
translatable="yes">Play</property>
                   </object>
                 </child>
               </object>


Second I have tried adding accessible child to the image like this 
however this is still inaccessible while using orca:

               <object class="GtkToolButton" id="play-button">
                 <property name="use_action_appearance">False</property>
                 <property name="related_action">play-action</property>
                 <property name="visible">True</property>
                 <property name="can_focus">False</property>
                 <property name="icon_widget">play-image</property>
               </object>

and somewhere else where the image is found in the definitions...

   <object class="GtkImage" id="play-image">
     <property name="visible">True</property>
     <property name="can_focus">False</property>
     <property name="icon_name">media-playback-start-symbolic</property>
     <child internal-child="accessible">
       <object class="AtkObject" id="play-image-atkobject">
         <property name="AtkObject::accessible-name" translatable="yes" 
context="Media controls">Play</property>
       </object>
     </child>
   </object>


Unfortunatelly while testing both the examples with orca the button is 
not accessible. Can you please explain the issue to me and / or point me 
to where I can study why this is failing for me?

Greetings

Peter