Utilize bottom portion of AdwToolbarView

Suggested by Pedro Paulo. This doesn't make much difference to the UI,
but it works and it's nice, so why not?

data/chess-window.ui

@@ -124,7 +124,9 @@
                 </child>
               </object>
             </child>
-            <child>
+          </object>
+	</property>
+	<child type="bottom">
           <object class="GtkBox" id="navigation_box">
             <property name="can-focus">0</property>
             <property name="spacing">6</property>
@@ -255,8 +257,6 @@
           </object>
 	</child>
       </object>
-        </property>
-      </object>
     </child>
   </template>
   <object class="GtkSizeGroup">